vc4: Speed up glGenerateMipmaps by avoiding shadow baselevel.

vc4: Speed up glGenerateMipmaps by avoiding shadow baselevel.

To support general GL_TEXTURE_BASE_LEVEL we have to copy to a temporary
miptree.  However, if a single level is being selected, we can use the
existing miptree and force all the sampling to be from that particular
level.

This avoids a ton of software fallbacks in glGenerateMipmaps(), which uses
base levels in the blit implementation in gallium.  Improves "glmark2 -b
terrain" from 2 fps to 3 (perhaps some more precision would be useful?),
and cuts its CPU usage during the benchmarking from ~30% to ~10% (total
CPU time from 8.8s to 7.6s).