fb: in viv_gpu_top add compile time option to use clock_

monotonous time should be more appropriate for a profiling tool,
however it is not always available
driver: fix scissor state in explicit context

This commit changes scissor state in reset to _FIXP.
driver: zero-initialize TS memory

It is important to initialize the TS to zero, as random pattern
can result in crashes. Do this on the CPU as this only happens once
per surface anyway and it's a small area, so it may not be worth
queuing this to the GPU.