drm/i915: Rename intel_output to intel_encoder.

The intel_output naming is inherited from the UMS code, which had a
structure of screen -> CRTC -> output.  The DRM code has an additional
notion of encoder/connector, so the structure is screen -> CRTC ->
encoder -> connector.  This is a useful structure for SDVO encoders
which can support multiple connectors (each of which requires
different programming in the one encoder and could be connected to
different CRTCs), or for DVI-I, where multiple encoders feed into the
connector for whether it's used for digital or analog.  Most of our
code is encoder-related, so transition it to talking about encoders
before we start trying to distinguish connectors.

This patch is produced by sed s/intel_output/intel_encoder/ over the
driver.

Signed-off-by: Eric Anholt <eric@anholt.net>