gma500: Fix symbol clash with i915

Randy Dunlap reports:
| when both CONFIG_DRM_I915=y and CONFIG_DRM_PSB=y:
| drivers/staging/built-in.o: In function `intel_opregion_init':
| (.text+0x47943): multiple definition of `intel_opregion_init'
| drivers/gpu/built-in.o:(.text+0x17277a): first defined here

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>