MLK-18177-1: drm/panel: Move the DSI init into enable stage

MLK-18177-1: drm/panel: Move the DSI init into enable stage

Currently, the DSI panel init sequence is made in the prepare function,
right after the reset pin is asserted. This implies that at this moment,
the DSI host needs to be enabled. If the DSI host is enabled during
panel prepare, there will be DSI signal on the DSI lanes during the
panel reset, which is wrong.

In order to not to have any signal on the DSI data lanes during reset,
the reset sequence must be separated from the init sequence, so move the
init into enable function and leave the reset into the prepare function.

Also:
- removed the calls to panel_disable and panel_unprepare from
  panel_remove, since the panel should be already disabled when this call
  is made
- fixed the call ordering to panel_disable and panel_unprepare from
  rad_panel_shutdown function

Signed-off-by: Robert Chiras <robert.chiras@nxp.com>