drm/msm/adreno: deal with linux-firmware fw paths

When firmware was added to linux-firmware, it was put in a qcom sub-
directory, unlike what we'd been using before.  For a300_pfp.fw and
a300_pm4.fw symlinks were created, but we'd prefer not to have to do
this in the future.  So add support to look in both places when
loading firmware.

Signed-off-by: Rob Clark <robdclark@gmail.com>