drm/radeon: fix a bogus kfree

parser->chunks[.].kpage[.] is not always kmalloc-ed
by the parser initialization, so parser_fini should
not try to kfree it if it didn't allocate it.

This patch fixes a kernel oops that can be provoked
in UMS mode.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>