v3d: take into account prim_counts_offset

Specifically when reading the primitive counters.

This fixed ~700 CTS tests using this pattern:
dEQP-GLES3.functional.transform_feedback.*

when run after tests like
dEQP-GLES3.functional.prerequisite.read_pixels on the same
caselist. When run individually those tests were passing because
prim_counts_offset was zero.

Fixes: 0f2d1dfe ("v3d: use the GPU to
       record primitives written to transform feedback")

Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
(cherry picked from commit fa41a518)