drm: Remove __OS_HAS_AGP

We already express the drm/agp depencies correctly in Kconfig, so we
can rip this remnant from the shared drm core days.

Aside: Pretty much all the #ifdefs in radeon/nouveau could be killed
if ttm would provide dummy functions. I'm not going to volunteer for
that though.

v2: Use IS_ENABLED(CONFIG_AGP) as suggested by Ville

v3: Polish from Ville's review.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com> (v2)
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_agpsupport.c

@@ -36,8 +36,6 @@
 #include <linux/slab.h>
 #include "drm_legacy.h"
 
-#if __OS_HAS_AGP
-
 #include <asm/agp.h>
 
 /**
@@ -502,5 +500,3 @@ drm_agp_bind_pages(struct drm_device *dev,
 	return mem;
 }
 EXPORT_SYMBOL(drm_agp_bind_pages);
-
-#endif /* __OS_HAS_AGP */