drm: rcar-du: lvds: Fix post-DLL divider calculation

drm: rcar-du: lvds: Fix post-DLL divider calculation

The PLL parameters are computed by looping over the range of acceptable
M, N and E values, and selecting the combination that produces the
output frequency closest to the target. The internal frequency
constraints are taken into account by restricting the tested values for
the PLL parameters, reducing the search space. The target frequency,
however, is only taken into account when computing the post-PLL divider,
which can result in a 0 value for the divider when the PLL output
frequency being tested is lower than half of the target frequency.
Subsequent loops will produce a better set of PLL parameters, but for
some of the iterations this can result in a division by 0.

Fix it by clamping the divider value. We could instead restrict the E
values being tested in the inner loop, but that would require additional
calculation that would likely be less efficient as the E parameter can
only take three different values.

Fixes: c4d86ae5336c ("drm: rcar-du: lvds: D3/E3 support")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>