drm: missing init_mm symbol, compatibility fix

The drm_compat.c version of kmap_atomic_prot_pfn() uses the macro
pgd_offset_k(), which references the symbol init_mm.
Starting in 2.6.25, init_mm is no longer exported by default.

The only user of kmap_atomic_prot_pfn() is i915, so this should
not hurt anyone, and it allows people to load drm.ko.

Signed-off-by: Pekka Paalanen <pq@iki.fi>