nv50: always keep TSC slot 0 bound

nv50: always keep TSC slot 0 bound

All TXF operations implicitly use sampler 0, and fail if it's not bound
to anything. This does not happen in LINKED_TSC mode, but we don't
currently use this.

We ensure that TSC entry at id 0 has the SRGB conversion bit enabled
(and all samplers we normally generate will too). Then when the TSC at
*slot* 0 (not to be confused with entry 0 in the global TSC table) is
unbound, we bind it to entry 0. This way, TXF operations are not
dependent on there being a regular sampler bound there.

Fixes arb_texture_buffer_object-subdata-sync among others. (TBO's are
particularly susceptible to this as they don't bind a sampler.)

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