drm/vmwgfx: Fix compiler warning with 32-bit dma_addr_t

When the size of dma_addr_t was 32 bits, the compiler warned
about the size of the 32 bit shift being larger than the size
of the data type.

Reported by Intel's kbuild robot.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>