clk: qcom: Allow clk_set_parent() to work on display clocks

Sometimes the display driver may want to change the parent PLL of
the display clocks (byte and pixel clocks) depending on the
use-case. Currently the parent is fixed by means of having a
frequency table with one entry that chooses a particular parent.
Remove this restriction and use the parent the clock is
configured for in the hardware during clk_set_rate(). This
requires consumers to rely on the default parent or to configure
the parent with clk_set_parent()/assigned-clock-parents on the
clocks before calling clk_set_rate().

Tested-by: Archit Taneja <architt@codeaurora.org>
Cc: Hai Li <hali@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>