drm/vmwgfx: Cleanup fifo mmio handling

Going forward the svga device might reuse mmio for general
register accesses, in order to prepare for that we need to
cleanup our naming and handling of fifo specific mmio reads
and writes. As part of this work lets switch to managed
mapping of the fifo mmio to make the error handling cleaner.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/414045/?series=85516&rev=2