v3d: fix indirect BO allocation for uniforms

v3d: fix indirect BO allocation for uniforms

We were always ensuring a minimum size of 4 bytes for uniforms
for the case where we don't have any, to account for hardware pre-fetching
of the uniform stream, however, pre-fetching could also lead to to out
of bounds reads when have read the last uniform in the stream, so we
probably want to have the extra 4 bytes to prevent the kernel from
observing invalid memory accesses when the uniform stream sits right at
the end of a page.

This seems to fix MMU exceptions reported with a Linux 5.4 kernel.

Credit goes to Phil Elwell for identifying the problem and narrowing
it down to memory accesses in the uniform stream.

Reported-by: Phil Elwell <phil@raspberrypi.org>
Tested-by: Phil Elwell <phil@raspberrypi.org>
Reviewed-by: Eric Anholt <eric@anholt.net>