msm: kgsl: Check for an OPP table without accidentally creating one

Commit 789e0443 ("msm: kgsl: Don't populate the OPP table if it
already exists") tried to avoid duplicating the OPP table but used
dev_pm_opp_get_opp_table() which creates a table if it didn't already
exist so the if() check was always true and we never populated the
table which in turn caused devfreq to fail because the table was empty.

All we really wanted to do was check to see if the table existed
without a nasty message from the OPP subsystem. Checking
dev_pm_opp_get_opp_count() should do that without any ugly side effects.

Change-Id: Ic0dedbadf81a1917b756144e762a6600a84863ca
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>