Build break due to 28ec711 "drm/agp: move AGP cleanup paths to drm_agpsupport.c"

[Stephen Warren]

In next-20130808, building tegra_defconfig for ARM yields:

> drivers/built-in.o: In function `drm_lastclose':
> /home/swarren/shared/git_wa/kernel/kernel.git/drivers/gpu/drm/drm_drv.c:198: undefined reference to `drm_agp_clear'

That's because drm_agp_clear() is called unconditionally, yet is only
conditionally built into drm_agpsupport.c (#if __OS_HAS_AGP).

Should the call from drm_drv.c be conditional, or should there be a
dummy static inline replacement in include/drm/drmP.h for when AGP
support isn't available?