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.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Acked-by: Mark Yao <mark.yao@rock-chips.com>