drm/i915: Move GT registers to their own header file

This is a huge, chaotic mass of registers copied over as-is without any
real cleanup.  We'll come back and organize these better, align on
consistent coding style, remove dead code, etc. in separate patches
later that will be easier to review.

v2:
 - Add missing include in intel_pxp_irq.c
v3:
 - Correct a few indentation errors (Lucas)
 - Minor conflict resolution

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220127234334.4016964-6-matthew.d.roper@intel.com