Previously we set only the dma mask and not the coherent mask. Fix that.
Also, for clarity, make sure both are initially set to 64 bits.

Cc: <stable@vger.kernel.org>
Fixes: 0d00c488: ("drm/vmwgfx: Fix the driver for large dma addresses")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>