drm/vmwgfx: Fix multiple command buffer context use

The start / stop and preempt commands don't honor the context argument
but rather acts on all available contexts.

Also add detection for context 1 availability.

Note that currently there's no driver interface for submitting buffers
using the high-priority command queue (context 1).

Testing done:
Change the default context for command submission to 1 instead of 0,
verify basic desktop functionality including faulty command injection and
recovery.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>