drm/nouveau/kms/nv50-: Store the bpc we're using in nv50_head_atom

In order to be able to use bpc values that are different from what the
connector reports, we want to be able to store the bpc value we decide
on using for an atomic state in nv50_head_atom and refer to that instead
of simply using the value that the connector reports throughout the
whole atomic check phase and commit phase. This will let us (eventually)
implement the max bpc connector property, and will also be needed for
limiting the bpc we use on MST displays to 8 in the next commit.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 232c9eec ("drm/nouveau: Use atomic VCPI helpers for MST")
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@redhat.com>
Cc: Jerry Zuo <Jerry.Zuo@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Juston Li <juston.li@intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: <stable@vger.kernel.org> # v5.1+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>