msm: kgsl: Handle error from nvmem_cell_read while reading speed_bin fuse

nvmem_cell_read can return ERR_PTR in case of failure. Current code is not
handling the error case. This would result in using default clock plan as
speed_bin value is zero. This can be fatal in case fuse was blown to a
lower frequency then the default clock plan. Handle this error to avoid
this issue.

Change-Id: Ic128265fdd1adefbbc0d7f733ef3e9b2f6fcb31a
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>