nvc0: fix a warning -Wconstant-conversion

I do not understand the code here well enough to tell what the correct
behavior is. prog->num_gprs is a unit8_t, so my guess is that the MIN
is there to make sure we stay within the limit of that. However the
current logic is a bit strange. If info_out.bin.maxGPR + 5 is bellow
256 we use that. If not then we write 256 which converts to 0 in a

The patch changes the upper value to 255.

Fixes a warning with clang

Reviewed-by: Karol Herbst <kherbst@redhat.com>
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