vc4: Drop maximum number of varyings down to 8.

vc4: Drop maximum number of varyings down to 8.

There are only 32 bits in the flatshade flags (which are 1 bit per
component), the simulator crashes when you use more than about this many
varyings, and the original Broadcom code drop only exposed 8 as well.

Fixes 26 piglit tests in the varying-packing group, and makes many others
go from crash to fail (due to not checking their varying counts and
treating link failures as failures).  Regresses ARB_fp/minmax (due to 8
varyings instead of 10).