drm/i915: Extract display registers from i915_reg.h to display

drm/i915: Extract display registers from i915_reg.h to display

There are certain register definitions which are defined in i915_reg.h
which are exclusively needed by display. Move the same to display
headers to remove i915_reg.h includes from display. This is a step
towards making display independent of i915.

intel_clock_gating.c can include display header directly, since its
usage is planned to be re-factored and will be moved within display.

v3: Updated subject and commit message (Jani)

v2: Drop common header in include and use display_regs.h (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patch.msgid.link/20260205094341.1882816-2-uma.shankar@intel.com