drm/xe/guc: Eliminate RPe caching for SLPC parameter handling

drm/xe/guc: Eliminate RPe caching for SLPC parameter handling

RPe is runtime-determined by PCODE and caching it caused stale values,
leading to incorrect GuC SLPC parameter settings.
Drop the cached rpe_freq field and query fresh values from hardware
on each use to ensure GuC SLPC parameters reflect current RPe.

v2: Remove cached RPe frequency field (Rodrigo)
v3: Remove extra variable (Vinay)
    Modify function name (Vinay)
v4: Maintain a separate function for PVC (Rodrigo)
v5: Avoid RPn update while fetching RPe frequency (Rodrigo)
v6: Split platform-specific RPe comments (Vinay)

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/5166
Signed-off-by: Sk Anirban <sk.anirban@intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://patch.msgid.link/20251112185153.3593145-5-sk.anirban@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>