msm: clock-rpm: Correct is_enabled implementation for RPM-SMD clocks

Clocks using the RPM-SMD driver have no way to query
clock state from the RPM. Therefore do not use the
get_rate function to figure out the enable-state of
the clock.

Change-Id: I4d142c716a4e41e54185e4ed4fe9feb33a1c2427
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>