Use drm_mode_get_hv_timing() to fill out the plane clip rectangle.

Note that this replaces crtc_state->adjusted_mode usage with
crtc_state->mode. The latter is the correct choice since that's the
mode the user provided and it matches the plane crtc coordinates
the user also provided.

Once everyone agrees on this we can move the clip handling into
drm_atomic_helper_check_plane_state().

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180123170857.13818-2-ville.syrjala@linux.intel.com
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com> #irc