drm/xe: Remove unused "mmio_ext" code

drm/xe: Remove unused "mmio_ext" code

The "mmio_ext" and 'REG_EXT" code is currently unused on any existing
platform.  Going forward, this also isn't the design we want to use for
any future platforms/features either, so we should just go ahead and
remove the dead code to avoid confusion.

mmio_ext was originally added in an attempt to hack around the early
(mis)design of the Xe driver, which used xe_gt as the target for all
register MMIO access, even those completely unrelated to the GT subunit
of the hardware.  With the introduction of commit 34953ee349dd ("drm/xe:
Create dedicated xe_mmio structure") and its follow-up patches, that
misdesign has been corrected and access to register MMIO regions
specific to hardware units is now done through xe_mmio structures which
encapsulate an iomap, region size, and some other metadata.

Although all of the registers used by the driver today happen to fall
within one specific PCI BAR region, and thus re-use a single device-wide
iomap, there's no requirement that this stay true for future platforms
or features.  I.e., if a future platform adds a new 'foo' hardware unit
that exists at a different area in the BAR, or even in a completely
different BAR, then that would be handled by doing a separate iomap of
that unit's register region and wrapping it in its own 'struct xe_mmio
foo_regs' structure.  The pointer to the new 'foo_regs' could be placed
within the xe_device, xe_tile, xe_gt, etc., according to where the new
hardware unit falls within the current hardware hierarchy.

This effectively reverts the following commits, although parts of these
commits had already vanished or changed with the earlier xe_mmio
refactor work:

 - commit 399a13323f0d ("drm/xe: add 28-bit address support in struct
   xe_reg")
 - commit fdef72e02e20 ("drm/xe: add a flag to bypass multi-tile config
   from MTCFG reg")
 - commit 866b2b176434 ("drm/xe: add MMIO extension support flags")
 - commit ef29b390c734 ("drm/xe: map MMIO BAR according to the num of
   tiles in device desc")
 - commit a4e2f3a299ea ("drm/xe: refactor xe_mmio_probe_tiles to support
   MMIO extension")

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Koby Elbaz <kelbaz@habana.ai>
Acked-by: Maciej Patelczyk <maciej.patelczyk@intel.com>
Reviewed-by: Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250106234312.2986065-2-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>