clk: msm: mdss: fix calculation of VCO rate during handoff

When continuous splash screen feature is enabled, the DSI PLL
is configured in the bootloader and left on when the kernel boots
up. When the PLL clocks are handed off, the VCO rate needs to be
computed back from the registers. Fix bugs in the current
implementation for calculating the VCO rate.

CRs-Fixed: 1037857
Change-Id: I8905b91f26a66d26959fb109480f0390851cbdb4
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>