gma500: Fix suspend/resume functions

Both the suspend and resume functions incorrectly set psbfb =
to_psb_fb(NULL) outside of the loop over all of the framebuffers. Fix
this by moving the assignment of psbfb inside the loop and removing the
initialisation of fb.

Signed-off-by: Ryan Mallon <rmallon@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>