drm/i915/display: split out intel_fbc_regs.h from i915_reg.h

Clean up i915_reg.h.

v2: Drop chicken regs and comments (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/aa9b5d8adefbe97e1e37c9cfada3ab1581b0e8d5.1714128645.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>