drm_compat: remove kmap_atomic_prot_pfn()

This function is unused, and yet creates build problems: the symbol
init_mm is not exported by the latest -rc kernels and I don't believe it
ever will be. Even CONFIG_UNUSED_SYMBOLS does not provide it anymore.

If this function is needed in the future, it needs to be reinvented in
any case. So remove it.

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