drm/xe/display: Create a dummy version for vga decode

This introduces an exclusive version of vga decode for xe.
Rest of the display changes will be re-used from i915.

Currently it adds just a dummy implementation. VGA decode
needs to be handled correctly in i915, proper implementation
will be adopted once the i915 changes are finalized and merged
in upstream.

v2: Addressed Arun's review comments

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Arun R Murthy <arun.r.mruthy@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>