drm/ast: Fix incorrect register check for DRAM width

During DRAM initialization on certain ASpeed devices, an incorrect
bit (bit 10) was checked in the "SDRAM Bus Width Status" register
to determine DRAM width.

Query bit 6 instead in accordance with the Aspeed AST2050 datasheet v1.05.

Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>