drm/msm/a5xx: Always set an OPP supported hardware value

If the opp table specifies opp-supported-hw as a property but the driver
has not set a supported hardware value the OPP subsystem will reject
all the table entries.

Set a "default" value that will match the default table entries but not
conflict with any possible real bin values. Also fix a small memory leak
and free the buffer allocated by nvmem_cell_read().

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Rob Clark <robdclark@chromium.org>