broadcom/vc5: Fix UIF surface size setup for ARB_fbo's mismatched sizes.

broadcom/vc5: Fix UIF surface size setup for ARB_fbo's mismatched sizes.

The HW was computing an implicit height for the surface based on the image
size, but that may be smaller than the surface with ARB_fbo mismatched
sizes.  In that case, we need to tell it about the pad, either with the
little 4-bit field in the RT config, or the extended field in
CLEAR_COLORS_PART3.

Fixes piglit arb_framebuffer_object-mixed-buffer-sizes.