drm/intel: add pick.h for the various "picker" helpers

drm/intel: add pick.h for the various "picker" helpers

Add a shared header that's used by i915, xe, and i915 display.

This allows us to drop the compat-i915-headers/i915_reg_defs.h include
from xe_reg_defs.h. All the register macro helpers were subtly pulled in
from i915 to all of xe through this.

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patch.msgid.link/fcd70f3317755bf98a6e7ae88974aa8ba06efd1e.1772042022.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>