turnip: fix tu6_emit_blit_scissor() when render area has zero size

As render_area->extent.{width,height} - 1 will be negative, the stored value will be the two's complement value of it, UINT_MAX in this case.

As a consequence, the assert checking the field values in A6XX_RB_BLIT_SCISSOR_BR will fail.

Fixes dEQP test:

dEQP-VK.api.pipeline.renderpass.framebuffer_compatible_renderpass

Signed-off-by: Samuel Iglesias Gonsálvez siglesias@igalia.com