drm/nouveau/ltc/gm200: limit NV_MMU_PTE_COMPTAGLINE bits to 16 where required

If NV_PFB_MMU_CTRL_USE_FULL_COMP_TAG_LINE is TRUE, then the last bit of
NV_MMU_PTE_COMPTAGLINE is re-purposed to select the upper/lower half of
a compression tag when using 64KiB big pages.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>