powerpc/ps3: Replace the flip_ctl logic in ps3av and ps3fb by a mutex

Introduce ps3_gpu_mutex to synchronizes GPU-related operations, like:
  - invoking the L1GPU_CONTEXT_ATTRIBUTE_FB_BLIT command using the
    lv1_gpu_context_attribute() hypervisor call,
  - handling the PS3AV_CID_AVB_PARAM packet in the PS3 A/V Settings driver.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>