drm/panel: add .prepare() and .unprepare() functions

Panels often require an initialization sequence that consists of three
steps: a) powering up the panel, b) starting transmission of video data
and c) enabling the panel (e.g. turn on backlight). This is usually
necessary to avoid visual glitches at the beginning of video data
transmission.

Similarly, the shutdown sequence is typically done in three steps as
well: a) disable the panel (e.g. turn off backlight), b) cease video
data transmission and c) power down the panel.

Currently drivers can only implement .enable() and .disable() functions,
which is not enough to implement the above sequences. This commit adds a
second pair of functions, .prepare() and .unprepare() to allow more
fine-grained control over when the above steps are performed.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
[treding: rewrite changelog, add kerneldoc]
Signed-off-by: Thierry Reding <treding@nvidia.com>