drm/panfrost: Simplify devfreq utilisation tracking

Instead of tracking per-slot utilisation track a single value for the
entire GPU. Ultimately it doesn't matter if the GPU is busy with only
vertex or a combination of vertex and fragment processing - if it's busy
then it's busy and devfreq should be scaling appropriately.

This also makes way for being able to submit multiple jobs per slot
which requires more values than the original boolean per slot.

Signed-off-by: Steven Price <steven.price@arm.com>