clk: msm: mdss: fix DSI PLL programming for msm8998

VCO configuration should be based on the requested vco
clock rate and should not factor in the bit clock source
divider. In addition, the bit clock source divider for
the slave controller should always be set to 1. This will
ensure that the PLL is locked at the correct rate.

CRs-Fixed: 1019289
Change-Id: Ie5c171e13dcccc711ba03acb38fcd7876e792cee
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>