drm/{i915, xe}/pcode: move display pcode calls to parent interface

drm/{i915, xe}/pcode: move display pcode calls to parent interface

Call the parent driver pcode functions through the parent interface
function pointers instead of expecting both to have functions of the
same name.

In i915, add the interface to existing intel_pcode.[ch], while in xe
move them to new display/xe_display_pcode.[ch] and build it only for
CONFIG_DRM_XE_DISPLAY=y.

Do not add separate write and write_timeout calls in the
interface. Instead, handle the default 1 ms timeout in the
intel_parent.c glue layer.

This drops the last intel_pcode.h includes from display, and allows us
to remove the corresponding xe compat header.

v2: initialize .pcode in i915

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Link: https://patch.msgid.link/20260126112925.2452171-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>