Bspec says:
"Restriction : SRD must not be enabled when the PSR Setup time from DPCD
00071h is greater than the time for vertical blank minus one line."

Let's check for that and disallow PSR if we exceed the limit.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>