msm_fb: display: provide panel refresh rate information

user space can use refresh rate of the panel when they get
variable screen info as part of calling FBIOGET_VSCREENINFO
ioctl.

CRs-Fixed: 283750
Signed-off-by: Rajesh Sastrula <vrajesh@codeaurora.org>
(cherry picked from commit 9322f75)

Change-Id: Ie05feb6dcc8ff660c04bcd4cc76056e625b61e30
Signed-off-by: Ramakrishna Prasad N <crpn@codeaurora.org>