drm: create gitignore file for radeon

Got lots of untracked files after compiling.
These files are generated, thus should be ignored by git.
Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/.gitignore

+mkregtable
+*_reg_safe.h
+