drm: Add atomic variants of enable/disable to encoder helper funcs

This patch adds atomic_enable and atomic_disable callbacks to the
encoder helpers. This will allow encoders to make informed decisions in
their start-up/shutdown based on the committed state.

Aside from the new hooks, this patch also introduces the new signature
for .atomic_* functions going forward. Instead of passing object state
(well, encoders don't have atomic state, but let's ignore that), we pass
the entire atomic state so the driver can inspect more than what's
happening locally.

This is particularly important for the upcoming self refresh helpers.

Changes in v3:
- Added patch to the set
Changes in v4:
- Move atomic_disable above prepare (Daniel)
- Add breadcrumb to .enable() docbook (Daniel)
Changes in v5:
- None
Changes in v6:
- Tweak kerneldoc some more (Sam)

Link to v3: https://patchwork.freedesktop.org/patch/msgid/20190502194956.218441-2-sean@poorly.run
Link to v4: https://patchwork.freedesktop.org/patch/msgid/20190508160920.144739-2-sean@poorly.run
Link to v5: https://patchwork.freedesktop.org/patch/msgid/20190611160844.257498-2-sean@poorly.run



Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611204959.180855-1-sean@poorly.run