It's better to catch such fallout early, and this way we can rely on
the checking done by the drm core on fb->heigh/width at modeset time.

If we ever support planar formats on intel we might want to look into
a common helper to do all this, but for now this is good enough.

v2: Take tiling into account, requested by Ville.

v3: Fix tile height on gen2, spotted by Ville.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Requested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>