freedreno/a6xx: Set a level's pitch based on minified level0 pitch, not width0.

freedreno/a6xx: Set a level's pitch based on minified level0 pitch, not width0.

Found from piglit fbo-generatemipmaps failures, then tracked down with the
texturator test.  The piece that really revealed things was finding that
1024x1 linear RGBA8 on the older blob drivers would have a pitch of 5120
instead of 4096, and the following levels minified that pitch.

Fixes ~124 piglit tests (~8.5% of piglit failures) on cheza.

Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3987>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3987>