drm/msm: Fix spelling "purgable" -> "purgeable"

The previous patch fixes the user visible spelling.  This one fixes the
code.  Oops.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210406151816.1515329-1-robdclark@gmail.com


Signed-off-by: Rob Clark <robdclark@chromium.org>