v3d: add shader-db stat to count SFU stalls

v3d: add shader-db stat to count SFU stalls

SFU operations have a latency of 2 cicles, so if their results
are used in the following cycle to a SFU instruction, the GPU
stalls for an extra cycle until the result is available.

This adds the number of stalls to the shader-db debug mode and
sum of instruction + stalls to evaluate optimizations to schedule
instructions that avoid generating sfu-stalls.

v2: Rename v3d_qpu_generates_sfu_stalls to v3d_qpu_instr_is_sfu (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>