drm/vmwgfx: Cleanup the cmd/fifo split

Lets try to cleanup the usage of the term FIFO which we used for
both our MMIO based cmd queue processing and for general
command processing which could have been using command buffers
interface. We're going to rename the functions which are processing
commands (and work either via MMIO or command buffers) as _cmd_
and functions which operate on the MMIO based commands as FIFO
to match the SVGA device naming.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/414044/?series=85516&rev=2