drm/rcar-du: dsi: Clean up VCLK divider calculation

Currently, in rcar_mipi_dsi_parameters_calc(), the VCLK divider is stored
in setup_info structure as BIT(divider). The rcar_mipi_dsi_parameters_calc()
is called at the early beginning of rcar_mipi_dsi_startup() function. Later,
in the same rcar_mipi_dsi_startup() function, the stored BIT(divider) value
is passed to __ffs() to calculate back the divider out of the value again.

Factor out VCLK divider calculation into rcar_mipi_dsi_vclk_divider()
function and call the function from both rcar_mipi_dsi_parameters_calc()
and rcar_mipi_dsi_startup() to avoid this back and forth BIT() and _ffs()
and avoid unnecessarily storing the divider value in setup_info at all.

This rework has a slight side-effect, in that it should allow the compiler
to better evaluate the code and avoid compiler warnings about variable
value overflows, which can never happen.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512051834.bESvhDiG-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202512222321.TeY4VbvK-lkp@intel.com/
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patch.msgid.link/20260105175250.64309-1-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

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