freedreno: Reduce use of screen->gpu_id

freedreno: Reduce use of screen->gpu_id

Newer GPU's are moving away from using gpu_id, including the code
landing upstream for "7c Gen 3".  But most of the places in the gallium
driver where we were looking at gpu_id, we only cared about the major
generation.  So convert those to use screen->gen instead.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12159>