drm/nv0x-nv4x: Leave the 0x40 bit untouched when changing CRE_LCD.

It's an unrelated PLL filtering control bit, leave it alone when
changing the CRTC-encoder binding.

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