drm/msm/mdp4: add LVDS panel support

LVDS panel support uses the LCDC (parallel) encoder.  Unlike with HDMI,
there is not a separate LVDS block, so no need to split things into a
bridge+connector.  Nor is there is anything re-used with mdp5.

Note that there can be some regulators shared between HDMI and LVDS (in
particular, on apq8064, ext_3v3p), so we should not use the _exclusive()
variants of devm_regulator_get().

The drm_panel framework is used for panel-specific driver.

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