clk: meson: fix rate calculation of plls with a fractional part

The rate of the parent should not be multiplied by 2 when the pll has a
fractional part. This is making the rate calculation of the gxl_hdmi_pll
wrong (and others as well). This multiplication is specific
to the hdmi_pll of gxbb and is most likely due to a multiplier sitting
in front of this particular pll.

Add a fixed factor clock in front on the gxbb pll and remove this constant
from the calculation to fix the problem

Fixes: 4a472951 ("clk: meson: fractional pll support")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>