gma500: fix ioctl confict

Move DRM_GMA_GET_PIPE_FROM_CRTC_ID to 0x08 to avoid confict with
DRM_GMA_GEM_MMAP.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>