msm: mdss: remove ctl power on check for getting mixer ids

While getting mixer ids attached to a particular control
path power_on need not be verified. The reason for power
on check was to prevent register programming during
splash updates, however register programming is taken
effect during the commit cycle.

Change-Id: I443300cbf5968c533628b8506f2c1ab13d55fca5
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>