drm: Add drm_connector_for_each_possible_encoder()

drm: Add drm_connector_for_each_possible_encoder()

Add a convenience macro for iterating connector->encoder_ids[].
Isolates the users from the implementation details.

Note that we don't seem to pass the file_priv down to drm_encoder_find()
because encoders apparently don't get leased. No idea why
drm_encoder_finc() even takes the file_priv actually.

Also use ARRAY_SIZE() when populating the array to avoid spreading
knowledge about the array size all over.

v2: Hide the drm_encoder_find() in the macro, and
    rename the macro appropriately (Daniel)
v3: Fix kernel docs (Daniel)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180628131315.14156-4-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>