drm/radeon: fix regression in UMS CS ioctl

commit 9b00147d9f2ba137ce74b66b768a8312be0b6781 upstream.

radeon_cs_parser_init is called by both the legacy UMS
CS ioctl and the KMS CS ioctl.  Protect KMS specific
pieces of the code by checking that rdev is not NULL.

Reported-by: Michael Burian <michael.burian@sbg.at>

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>