drm/vc4: Add support for MSAA rendering.

For MSAA, you set a bit in the binner that halves the size of tiles in
each direction, so you can pack 4 samples per pixel in the tile
buffer.  During rendering, you can load and store raw tile buffer
contents (to save the per-sample MSAA contents), or you can load/store
resolved tile buffer contents (loads spam the pixel value to all 4
samples, and stores either average the 4 color samples, or store the
first sample for Z/S).

Signed-off-by: Eric Anholt <eric@anholt.net>