drm/msm: Add the eDP connector in msm drm driver (V2)

Modified the hard-coded hdmi connector/encoder implementations in msm drm
driver to support both edp and hdmi.

V1: Initial change

V2: Address Thierry's change

Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>