drm/i915/mtl: do not enable render power-gating on MTL

Multiple CI tests fails with forcewake ack timeouts if render
power gating is enabled.
BSpec 52698 states it should be 0 for MTL, but apparently
this info is outdated. Anyway since the patch makes MTL pass basic
tests added FIXME tag informing this is temporary workaround.

v2: added FIXME tag

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/4983


Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230517-mtl_disable_render_pg-v2-1-0b51180a43f0@intel.com