drm/doc/rfc: Remove Xe's pre-merge plan

drm/doc/rfc: Remove Xe's pre-merge plan

The last TODO item here that was not marked as done was
the display portion, which came along with the pull-request.

So, now that Xe is part of drm-next and it includes the
display portion, let's entirely kill this RFC here.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240110190427.63095-1-rodrigo.vivi@intel.com
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>