drm/radeon/kms: fix regression is handling >2 heads on cedar/caicos

Need to add support for 4 crtcs when setting the possible crtcs
for the encoders.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>