The incoming mode might have a missing vrefresh field if it came from
drmModeSetCrtc(), which the kernel is supposed to calculate using
drm_mode_vrefresh().  We could either use that or the adjusted_mode's
original vrefresh value.

However, we can maintain a more exact vrefresh value (not just the
integer approximation), by scaling by the ratio of our clocks.

v2: Use math suggested by Andrzej Hajda instead.
v3: Simplify math now that adjusted_mode->clock isn't padded.
v4: Drop some parens.
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170815234722.20700-2-eric@anholt.netReviewed-by: Andrzej Hajda <a.hajda@samsung.com>