drm/msm/mdp4: Clean up some MDP4 clocks

Fix some issues with MDP4 clocks:

- mdp4_dtv_encoder tries to get "src_clk", which is a RCG(TV_SRC) in
  MSM8960 and APQ8064. This isn't something the driver should access or
  configure. Instead of this, configure the "mdp_clk" (MDP_TV_CLK), a
  branch clock in MMCC that has the TV_SRC as its parent. Setting
  rate/enabling the "mdp_clk" will eventually configure "src_clk", which
  is what we want.
- Rename "mdp_clk" to "tv_clk" because that's slightly less confusing.
- Rename "mdp_axi_clk" to "bus_clk" because that's what we do elsewhere
  too.

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