drm/i915: Add an update_pipe callback to intel_encoder and call this on fastsets (v2)

When we are doing a fastset (needs_modeset=false, update_pipe=true) we
may need to update some encoder-level things such as checking that PSR
is enabled.

This commit adds an update_pipe callback to intel_encoder and a new
intel_encoders_update_pipe helper which calls this for all encoders
connected to a crtc. The new intel_encoders_update_pipe helper is called
from intel_update_crtc when doing a fastset.

Changes in v2:
-Name the new encoder callback update_pipe instead of just update

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181220132120.15318-1-hdegoede@redhat.com