drm/nv50/disp: use correct register to determine DP display bpp

drm/nv50/disp: use correct register to determine DP display bpp

Commit ca477e4150 ("drm/nv50-/disp: move DP link training to core and
train from supervisor") added code that uses the wrong register for
computing the display bpp, used for bandwidth calculation. Adjust to use
the same register as used by exec_clkcmp and nv50_disp_intr_unk20_2_dp.

Reported-by: Torsten Wagner <torsten.wagner@gmail.com>
Reported-by: Michael Gulick <mgulick@mathworks.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67628
Cc: stable@vger.kernel.org # 3.9+
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>