msm_fb: display: fix frame rate wrong calculation

For cmd mode the frame rate calculation result is wrong. Also in mdp
driver there are several places to do the same calculation. So fix the
wrong calculation and use a single func to do calculation.

Change-Id: I3a4be39e480ce82677dcefb666202947f7f6d3c1
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
(cherry picked from commit d4eb5a1a3ef0935ce6aeffb2b48974c01b9518b9)
Signed-off-by: Neha Pandey <nehap@codeaurora.org>