drm/i915: merge soc/intel_gmch.[ch] to display/intel_vga.c

drm/i915: merge soc/intel_gmch.[ch] to display/intel_vga.c

The sole user of the remaining functions in intel_gmch.[ch] is in
intel_vga.c. Move everything there.

Since intel_gmch.c hasn't been part of xe, use a dummy function
relocated from xe_display_misc.c, with #ifdef. This is purely to keep
this change non-functional.

This allows us to remove soc/intel_gmch.[ch] from i915, compat
soc/intel_gmch.h from xe, and xe_display_misc.c from xe.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/b0f853ad7eae686738defa9e8f08a8848df8f226.1763578288.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>