drm/vmwgfx: Fix setting of dma masks

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>