drm: Do basic sanity checks for user modes

Currently userspace is allowed to pass in basiclly any kind of garbage
to setcrtc. Try to catch the cases where the timings make no sense
by passing the mode through drm_mode_validate_basic().

One concern here is that we now start to block some modes that have
worked in the past. It's at least possible with when using i915 with
LVDS/eDP. Previously we've just ignored everything but hdisplay/vdisplay
from the user mode and just overwritten the rest with the panel fixed
mode. So if someone has been passing a mode with just those populated
that would now stop working. If that is a real problem, we can't add
these checks to the core code and each driver would have to have its
own sanity checks. So fingers crossed...

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>