drm/i915/mchbar: move intel_mchbar_regs.h under include/drm/intel

drm/i915/mchbar: move intel_mchbar_regs.h under include/drm/intel

Since the mchbar registers are used from both i915 display and core,
move intel_mchbar_regs.h to include/drm/intel/mchbar_regs.h. Drop the
intel_ prefix from the name to reduce tautology.

With this, we can drop the corresponding xe display compat header.

v2: Rebase

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/6c951b2c05db74ea517d52a3912986f7eb886422.1775590536.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>