drm/msm: Centralize connector registration/unregistration

Move the drm_connector registration from the encoder(HDMI/DSI etc) drivers
to the msm platform driver. This will simplify the task of ensuring that
the connectors are registered only after the drm_device itself is
registered.

The connectors' destroy ops are made to use kzalloc instead of
devm_kzalloc to ensure that that the connectors can be successfully
unregistered when the msm driver module is removed. The memory for the
connectors is unallocated when drm_mode_config_cleanup() is called
during either during an error or during driver remove.

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