broadcom/vc5: Fix simulator assertion failures about color RT clears.

broadcom/vc5: Fix simulator assertion failures about color RT clears.

When we tried to clear color while storing depth, it assertion failed
about basically not having enough information to decide which color RT to
clear.  It turns out the STORE_GENERAL picks the buffer according to the
color buffer being stored, or all of them if NONE.  If you're doing depth,
it doesn't know which to pick.