drm/vmwgfx: switch from ioremap_cache to memremap

Per commit 2e586a7e "drm/vmwgfx: Map the fifo as cached" the driver
expects the fifo registers to be cacheable.  In preparation for
deprecating ioremap_cache() convert its usage in vmwgfx to memremap().

Cc: David Airlie <airlied@linux.ie>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>