drm/vmwgfx: Move irq bottom half processing to threads

This gets rid of the irq bottom half tasklets and instead performs the
work needed in process context. We also convert irq-disabling spinlocks to
ordinary spinlocks.

This should decrease system latency for other system components, like
sound for example but has the potential to increase latency for processes
that wait on the GPU.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>