nv50: add missing brackets when handling the samplers array

nv50: add missing brackets when handling the samplers array

Commit 3805a864b1d(nv50: assert before trying to out-of-bounds access
samplers) introduced a series of asserts as a precausion of a previous
illegal memory access.

Although it failed to encapsulate loop within nv50_sampler_state_delete
effectively failing to clear the sampler state, apart from exadurating
the illegal memory access issue.

Fixes gcc warning "array subscript is above array bounds" and
"Nesting level does not match indentation" and "Out-of-bounds read"
defects reported by Coverity.

Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>