drm/vmwgfx: Use enum to represent graphics context capabilities

Instead of having different bool in device private to represent
incremental graphics context capabilities, add a new sm type enum.

v2: Use enum instead of bit flag.

v3: Incorporated review comments.

Signed-off-by: Deepak Rawat <drawat.floss@gmail.com>
Reviewed-by: Thomas Hellström (VMware) <thomas_os@shipmail.org>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>