drm/i915: Use adjusted_mode in intel_update_fbc()

Check the mode flags from the adjusted_mode, not user requested mode.
The hdisplay/vdisplay check actually checkes the primary plane size,
so those still need to come from the user requested mode.

Extract both modes from pipe config instead of the drm_crtc.

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