drm/msm/mdp4: Call custom round_pixclk helper only if the encoder type is TMDS

The mdp_kms round_pixclk op creates problems when we have more
interfaces in use. It calls the DTV encoder's helper by default.

Check on encoder type and call the corresponding encoder's
func meant for rounding pixel clock. DSI and LVDS don't require
rounding, so just return rate in their case.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>