drm/i915/display: drop i915_reg.h include where possible

A number of files have unnecessary i915_reg.h includes. Drop them.

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Link: https://lore.kernel.org/r/7c4002322f4d8132fd2eaa1a4d688539cdd043c3.1749469962.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>