disp: msm: dsi: Add support to skip constant fps for command mode

VFP or HFP is adjusted to achieve constant fps during dynamic
DSI clock switch. This feature is not supported for command
mode. So, add check to skip porch calculation for command mode.

Change-Id: I5fa76d6536a55b2a19f24c0e14b6861e1f4c8f25
Signed-off-by: Lipsa Rout <lrout@codeaurora.org>