pvr: Remove unneeded assert from pvr_frag_state_stream_init()

Matt Coster requested to merge MTCoster/mesa:dev/no-ds-attach-support into main

What does this MR do and why?

Allow S8_UINT to be used as a stencil attachment format, the assert was just a relic at this point.

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Edited by Matt Coster

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