msm: mdss: dsi: fix panel minimum refresh rate configuration

In the current implementation, if a DSI panel does not explicitly
specify the minimum supported refresh rate, then it is set to
48 by default. This is incorrect since many panels may not support
that low of a refresh rate. Fix this by setting the default value
to the panel's actual refresh rate.

CRs-Fixed: 1056610
Change-Id: I8d4267528068e36a648c328fbe6d6a35943f3810
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>