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

Commit 0a0afd28 ("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>