msm: kgsl: Don't permit the user to set the lowest frequency

The user may set limits on the GPU frequency via sysfs nodes provided
by kgsl. When setting these limits, ignore the lowest GPU frequency
that is used only when disabling the GPU core clock.

Change-Id: Ie36ebfba1037d28b924ea706ebe2eb647e157367
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>