drm/radeon/kms: clip number of virtual crtcs

Total number of CRTCs per GPU cannot be greater than
32. If the driver loads with the parameter that requests
more CRTCs than we can handle (together with pre-existing
physical CRTCs) we clip it to maximum.

The side effect of this patch is that we can always load
the module with num_virtual_crtc=32 and get the maximum
possible number of virtual crtcs for a given GPU.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
vcrtcm: do not allow fps on unattached pcon

Allowing fps operation on a PCON that has not been
attached yet, can later result in some ugly kernel crashes.
This patch prevents such things from happening.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>