gma500: psb_intel_crtc: Drop crtc_enable flag.

This is set when setting DPMS on and off, but it isn't checked anywhere,
so just remove it.

Signed-off-by: Forest Bond <forest.bond@rapidrollout.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>