drm/i915/reg: make masked field helpers constexpr

drm/i915/reg: make masked field helpers constexpr

Make it possible to use _MASKED_FIELD(), _MASKED_BIT_ENABLE() and
_MASKED_BIT_DISABLE() in contexts that require integer constant
expressions. This increases their usefulness at the small cost of making
the warnings from build time checks less helpful.

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patch.msgid.link/788f538cc71dec0db25e0c768e8945bef6f9701c.1772042022.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>