drm/i915/pxp & drm/xe/pxp: Figure out pxp instance from the gem object

drm/i915/pxp & drm/xe/pxp: Figure out pxp instance from the gem object

It's undesirable to have to figure out the pxp pointer in display
code. For one thing, its type is different for i915 and xe.

Since we can figure the pxp pointer out in the pxp code from the gem
object, offload it there.

v2: Rebase

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250228114527.3091620-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>