drm/panel: ld9040: Add proper definition for prepare and unprepare

Move out code from enable and disable routines to prepare
and unprepare routines, so that functionality is properly
distributed across all the panel functions.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>