drm/nouveau: No need to lock/unlock the VGA CRTC regs all the time.

Locking only makes sense in the VBIOS parsing code as it's executed
before CRTC init.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>