drm/scheduler: track GPU active time per entity

Track the accumulated time that jobs from this entity were active
on the GPU. This allows drivers using the scheduler to trivially
implement the DRM fdinfo when the hardware doesn't provide more
specific information than signalling job completion anyways.

[Bagas: Append missing colon to @elapsed_ns]
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>