drm/msm/dsi: Don't get byte/pixel source clocks from DT

We retrieve the byte and pixel source clocks (RCG clocks) in the dsi
driver via DT. These are needed so that we can re-parent these source
clocks if we want to drive it using a different DSI PLL.

We shouldn't get these via DT because they aren't clocks that directly
serve as inputs to the dsi host.

Fortunately, there is a static parent-child link between the
byte_clk_src/pixel_clk_src and byte_clk/pixel_clk clocks. So, we can
retrieve the source clocks via clk_get_parent.

Do this instead of retrieving via DT.

Cc: Rob Herring <robh@kernel.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>