drm/i915: Unlink NV12 planes earlier

In the Linux kernel, the following vulnerability has been resolved:

drm/i915: Unlink NV12 planes earlier

unlink_nv12_plane() will clobber parts of the plane state
potentially already set up by plane_atomic_check(), so we
must make sure not to call the two in the wrong order.
The problem happens when a plane previously selected as
a Y plane is now configured as a normal plane by user space.
plane_atomic_check() will first compute the proper plane
state based on the userspace request, and unlink_nv12_plane()
later clears some of the state.

This used to work on account of unlink_nv12_plane() skipping
the state clearing based on the plane visibility. But I removed
that check, thinking it was an impossible situation. Now when
that situation happens unlink_nv12_plane() will just WARN
and proceed to clobber the state.

Rather than reverting to the old way of doing things, I think
it's more clear if we unlink the NV12 planes before we even
compute the new plane state.

(cherry picked from commit 017ecd04985573eeeb0745fa2c23896fb22ee0cc)