drm/omap: fix i886 work-around

7d267f06 ("drm/omap: work-around for
errata i886") changed how the PLL dividers and multipliers are
calculated. While the new way should work fine for all the PLLs, it
breaks omap5 PLLs. The issues seen are rather odd: seemed that the
output clock rate is half of what we asked. It is unclear what's causing
there issues.

As a work-around this patch adds a "errata_i886" flag, which is set only
for DRA7's PLLs, and the PLL setup is done according to that flag.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>