UPSTREAM: drm/rockchip: return a true clock rate to adjusted_mode

UPSTREAM: drm/rockchip: return a true clock rate to adjusted_mode

Since the mipi dsi driver need to use the clock of vop to make the
calculation of Blanking. But sometimes the clock driver can not set a
accurate clock_rate for vop, get it by clk_round_rate before mode_set,
so we can get the true value.

Change-Id: I04e6a499763258c2e16a09e3a59cf3a1e4593706
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
(cherry picked from commit b59b8de3149736e5094cb786978a1ba8d6d55b34)