vc4: fix vc4_yuv_blit overwriting fragment constant buffer slot 0

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.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2581
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3997>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3997>