drm/vmwgfx: Map the fifo as cached

On the guest kernel side, previously the FIFO has been mapped write-
combined. This has worked since VMs up to now has not honored the mapping
type and mapped the FIFO cached anyway. Since the FIFO is accessed cached
by the CPU on the virtual device side, this leads to inconsistent
mappings once the guest starts to honor the mapping types.

So ask for cached mappings when we map the FIFO. We do this by
using ioremap_cache() instead of ioremap_wc(), and remove the MTRR setup.
On the TTM side, MOBs, GMRs and VRAM buffers are already requesting
cached mappings for kernel- and user-space.

Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>