drm/xe: Fix name and definition of GFX_MODE register

drm/xe: Fix name and definition of GFX_MODE register

The register located at $base+0x29c is referred to as GFX_MODE in the
bspec.  Although many other registers have RING_* prefixes for
historical reasons, this register does not, so using a name that does
not match the bspec just makes it harder to recognize/find.

Also, GFX_MODE is a masked register (updating bits [15:0] requires that
the corresponding bit(s) in [31:16] are also set), so add the
XE_REG_OPTION_MASKED flag to the register definition; this will become
important when we start programming this register via RTP tables in a
future patch.

Finally swap the order of the register's two bit definitions to match
our regular coding style of descending order for register bits/fields.

Bspec: 45928
Reviewed-by: Shuicheng Lin <shuicheng.lin@intel.com>
Link: https://patch.msgid.link/20260424-engine-setup-v2-5-59cc620a25f1@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>