radeon/r200: fix bogus assert/scissor wrt width/height 2048

radeon/r200: fix bogus assert/scissor wrt width/height 2048

This addresses one issue raised in bug #51658 discovered by Eugene St Leger.
The assert is bogus since there's no problem with texture width/height being
2048 (the width/height programmed is width/height minus one).
OTOH though the programmed size for scissor rect should be width/height
minus one too otherwise bad things may happen (as it is inclusive, and there's
not enough bits for more than a value of 2047).