drm/xe: Clarify register types on PAT programming

Clarify a few things related to the PAT programming, particularly on
MTL:

	- The register type doesn't change depending on the GT - what
	  happens is that media GT writes to other set of registers that
	  are not MCR
	- Remove "UNICAST": otherwise it's confusing why it's not using
	  MCR registers with the unicast function variant

Also, there isn't much reason to keep those parts as macros: promote
them to proper functions and let the compiler inline if it sees fit.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230427223256.1432787-5-lucas.demarchi@intel.com


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