drm/i915/fbc: fix the implementation of wa_18038517565

drm/i915/fbc: fix the implementation of wa_18038517565

As per the wa_18038517565, we need to disable FBC compressor
clock gating before enabling FBC and enable after disabling
FBC. Placing the enabling of clock gating in the fbc deactivate
function can make the above wa logic go wrong in case of
frontbuffer rendering FBC mechanism. FBC deactivate can get
called during fb invalidate and then the corresponding FBC
activate can get called without properly disabling the clock
gating and can result in compression stalled. So move the
enable clock gating at the end of one FBC session after FBC
is completely disabled for a pipe.

Bspec: 74212, 72197, 69741, 65555
Fixes: 010363c46189 ("drm/i915/display: implement wa_18038517565")
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://lore.kernel.org/r/20250729124648.288497-1-vinod.govindapillai@intel.com