drm/i915/display: define and use intel_reg_{offset, equal, valid}() helpers

drm/i915/display: define and use intel_reg_{offset, equal, valid}() helpers

Add display specific helpers for getting the register offset, checking
for equality and validity. Add them as static inlines for increased type
safety.

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/7fe12d4e5465778209ccf29359767a197b031dd9.1777972548.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>