drm/i915: Add primary plane to atomic state in legacy modeset

Add the primary plane state to the legacy modeset atomic state and use
it when configuring the primary plane in __intel_set_mode(). This is a
first step towards merging the flip path in intel_crtc_set_config() and
__intel_set_mode().

v2: Set crtc to NULL if fb is NULL. (Maarten)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>