vmwgfx: Restrict number of GMR pages to device limit

When GMR2 is available, make sure we restrict the number of used GMR pages
to the limit indicated by the device.
This is done by failing a GMRID allocation if the total number of GMR pages
exceeds the limit.
As a result TTM will then start evicting buffers in GMR memory on a
LRU basis until the allocation succeeds.

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