When pixel clock less than 148.5MHz, make sloopboost=2 tklvl=20
cklvl=13 increase rasing/falling time and increase data & clock
voltage driver.

When pixel clock less than 74.25MHz, make sloopboost=0 tklvl=19
cklvl=18, increase data and clock voltage driver.
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>