drm/i915/reg: move masked field helpers to i915_reg_defs.h

This is a more logical place for generic helpers.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5161a0c6d98df206c6c4c1add3fc3f2f408020b1.1668008071.git.jani.nikula@intel.com