drm/i915: Clear most of crtc state when disabling the crtc

Currently we don't call intel_crtc_prepare_cleared_state() for crtcs
that are going to be entirely disabled (uapi.enable==false). That
means such crtcs will leave stale junk lying around in their states
and we have to sprinkle hw.enable checks all over before we can
look at the states. Let's change that a bit so that we aways do
the state clearing, even for fully disabled crtcs.

Note that we still keep some parts of the old state (see
intel_crtc_prepare_cleared_state() for the details) so probably
can't trust things 100% when hw.enable==false. But at least there's
less chance now that we end up looking at stale junk.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115190813.17971-2-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>