mfd: db8500-prcmu: Update stored DSI PLL divider value

Previously the DSI PLL divider rate was initialised statically and
assumed to be 1. Before the common clock framework was enabled for
ux500, a call to clk_set_rate() would always update the HW registers
no matter what the current setting was.

This patch makes sure the actual hw settings and the sw assumed
settings are matched.

Signed-off-by: Paer-Olof Haakansson <par-olof.hakansson@stericsson.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>