drm/radeon/kms: Fix module parameter description format

Module parameter descriptions don't take a trailing \n, otherwise it
breaks formatting of modinfo's output. Also add missing space after
comma.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: David Airlie <airlied@linux.ie>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>