drm/i915: Introduce intel_crtc_state_reset()

drm/i915: Introduce intel_crtc_state_reset()

We have a few places where we want to reset a crtc state to its
default values. Let's add a helper for that. We'll need the new
__drm_atomic_helper_crtc_state_reset() helper for this to allow
us to just reset the state itself without clobbering the
crtc->state pointer.

And while at it let's zero out the whole thing, except a few
choice member which we'll mark as "invalid". And thanks to this
we can now nuke intel_crtc_init_scalers().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107142417.11107-4-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>