nvc0: replace use of explicit default_tsc with entry 0

nvc0: replace use of explicit default_tsc with entry 0

This was used for implementing FBFETCH. However that uses TXF, which
doesn't do much with a TSC. The only important bit is that sRGB-decoding
works as expected, which we can achieve since all samplers we ever
generate enable sRGB-decoding. Always point to entry 0 in the TSC table,
and ensure that even before it ever gets initialized, the sRGB-decoding
enable bit is set.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>