drm/xe: Remove GEN[0-9]*_ prefixes

After noticing in logs there were still mentions to GEN6 registers, it
was clear commit d9b79ad2 ("drm/xe: Drop gen afixes from registers")
didn't take care of all the afixes. Some were added later, but there are
also constants and strings still using that.  Continue the cleanup
removing the remaining ones.

To keep it consistent with code nearby, a few other changes are made:

	- Remove prefix in INTEL_LEGACY_64B_CONTEXT
	- Remove GEN8_CTX_L3LLC_COHERENT since it's unused
	- Rename GEN9_FREQ_SCALER to GT_FREQUENCY_SCALER

v2: Use XELP_ as prefix for NUM_MOCS_ENTRIES and remove changes to
    MOCS_ENTRIES as this is now done as part of a previous commit
    (Matt Roper)

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20231117174049.527192-3-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>