msm: mdss: Don't call pan_idle for MSMFB_OVERLAY_PREPARE ioctl

We keep calling MSMFB_OVERLAY_PREPARE for each composition
cycle, even if there are no changes in params. Because of this
we can hit performance degradation due to pan idle wait.
This change removes pan_idle wait for prepare call.

CRs-Fixed: 600539
CRs-Fixed: 608848
Change-Id: Ie99f4dbc4671caf9bfbd49bd9b200c3180002d56
Signed-off-by: Radhika Ranjan Soni <rrsoni@codeaurora.org>