clk: tegra: Define PLLD_DSI and remove dsia(b)_mux

PLLD is the only parent for DSIA & DSIB on Tegra124 and
Tegra132. Besides, BIT 30 in PLLD_MISC register controls
the output of DSI clock.

So this patch removes "dsia_mux" & "dsib_mux", and create
a new clock "plld_dsi" to represent the DSI clock enable
control.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Mark Zhang <markz@nvidia.com>