drm/i915/fbc: keep FBC disabled if selective update is on in xe2lpd

FBC was disabled in case PSR2 selective update in display 12 to
14 as part of a wa. From xe2lpd onwards there is a logic to be
implemented to decide between FBC and selective update. Until
that logic is implemented keep FBC disabled in case selective
update is enabled.

v1: updated patch description and some explanation and todo

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250321094529.197397-2-vinod.govindapillai@intel.com