drm/msm/gem: Tidy up VMA API

Stop open coding VMA construction, which will be needed in the next
commit.  And since the VMA already has a ptr to the adress space, stop
passing that around everywhere.  (Also, an aspace always has an mmu so
we can drop a couple pointless NULL checks.)

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/527833/
Link: https://lore.kernel.org/r/20230320144356.803762-4-robdclark@gmail.com