drm/msm/mdp4: fix blend setup with multiple crtcs

In particular, blend_setup() should not overwrite the other crtc's mixer
settings.  Also, the encoder needs to be able to specify the mixer-id
explicitly, since both LVDS and DTV use 'INTF_LVDC_DTV', so we cannot
guess the mixer-id from the interface.

Signed-off-by: Rob Clark <robdclark@gmail.com>