drm: Implement timings adjustments for frame packing

When using the frame packing and a single big framebuffer, some hardware
requires that we do everything like if we were scanning out the big
buffer itself. Let's instrument drm_mode_set_crtcinfo() to be able to do
this adjustement if the driver is asking for it.

v2: Use crtc_vtotal and multiply the clock by 2 instead of
    reconstructing it (Ville Syrjälä)

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>