drm/i915: Allocate a crtc_state also when the crtc is being disabled

For consistency, allocate a new crtc_state for a crtc that is being
disabled. Previously only the enabled value of the current state would
change.

v2: Rebase on v5 of previous patch. (Ander)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
[danvet: Resolve rebase conflict.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>