vc4: fix vc4_yuv_blit overwriting fragment constant buffer slot 0

vc4_yuv_blit calls util_blitter_restore_constant_buffer_state without first calling util_blitter_save_fragment_constant_buffer_slot. This causes subsequent crashes in vc4_write_uniforms when using fragment shaders that reference YUV textures.

Closes: #2581 (closed)