drm/panel: Remove drm_panel_detach() calls from all panel drivers

Remove all drm_panel_detach() calls from all panel drivers and update
the kerneldoc for drm_panel_detach().

Setting the connector and drm to NULL when the DRM panel device is going
away hardly serves any purpose. Usually the whole memory structure is
freed right after the remove call. However, calling the detach function
from the master DRM device, and setting the connector pointer to NULL,
has the logic of marking the panel again as available for another DRM
master to attach. The usual situation would be the same DRM master
device binding again.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/464b8d330d6b4c94cfb5aad2ca9ea7eb2c52d934.1524727888.git.jsarha@ti.com