drm/xe: Do not spread i915_reg_defs.h include

Reduce the use of i915_reg_defs.h so it can be encapsulated in a single
place.

1) If it was being included by mistake, remove
2) If it was included for FIELD_GET()/FIELD_PREP()/GENMASK() and the
   like, just include <linux/bitfield.h>
3) If it was included to be able to define additional registers, move
   the registers to the relavant headers (regs/xe_regs.h or
   regs/xe_gt_regs.h)

v2:
  - Squash commit fixing i915_reg_defs.h include and with the one
    introducing regs/xe_reg_defs.h
  - Remove more cases of i915_reg_defs.h being used when all it was
    needed was linux/bitfield.h  (Matt Roper)
  - Move some  registers to the corresponding regs/*.h file (Matt Roper)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[Rodrigo squashed here the removal of the i915 include]