drm/vc4: Make sure to emit a tile coordinates between two MSAA loads.

The HW only executes a load once the tile coordinates packet happens,
and only tracks one at a time, so by emitting our two MSAA loads back
to back we would end up with an undefined color or Z buffer.

Fixes dEQP-EGL.functional.render.multi_context.gles2.rgb888_window

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190206232550.12012-1-eric@anholt.net