drm/vmwgfx: Fix a command submission hang regression

When we're out of command buffer space, we turn on the command buffer
processed irq without re-checking for finished command buffers afterwards.
This might lead to a missed irq and the command submission process waiting
forever for space.

Fix this by rerunning the command buffer submission handler whenever we're
out of command space. This ensures both that we don't needlessly turn on
the irq, and that if we decide to turn on the irq, we recheck for finished
command buffers before going to sleep.

Reported-and-tested-by: Bryan Li <ldexin@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>