drm/radeon: fix vm page table block size calculation

The page offset is 12 bits.  For example if we have an
8 GB VM, we'd need 33 bits.  The number of bits needed
for PD + PT is 21 (33 - 12 or log2(8) + 18), not 20
(log2(8) + 17).

Noticed by Alexey during code review.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org