We were promoting reads with a valid primary to direct CPU maps even
if the mmap mode was IRIS_MMAP_WC, which would mean uncached reads from
VRAM.  In that case, GPU blits are in fact useful!

We were also only checking for !DISCARD_RANGE rather than MAP_READ,
which isn't a great idea for image maps, given the discussion in the
previous commit about image map semantics.

The original code was also just confusingly structured.  Make a helper
function with clearly defined cases where we want to bail on CPU maps.