gpu: pvr: support for render complete events

gpu: pvr: support for render complete events

In DRM vertical blank event style, add support for userspace
to register for events signaling completion of rendering.
Driver is augmented with a list of pending events each
representing request to monitor a particular render completion.
Interrupt handler checks for the list and for the corresponding
state of the target surface. Once a complete render is
encountered, the handler simply moves the corresponding event
from the pending list to the unread list.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@nokia.com>
Signed-off-by: Imre Deak <imre.deak@nokia.com>