drm/msm/adreno/a5xx_debugfs: fix potential NULL pointer dereference

_minor_ is being dereferenced before it is null checked, hence there
is a potential null pointer dereference. Fix this by moving the pointer
dereference after _minor_ has been null checked.

Fixes: 024ad8df763f ("drm/msm: add a5xx specific debugfs")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>