vmwgfx: Break out execbuf command processing

This will make it easier to execute commands operating on user-space
resources but generated by the kernel.

JB: Added tracking if the sw_context was called from the kernel or userspace.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>