drm/meson: split out encoder from meson_dw_hdmi

This moves all the non-DW-HDMI code where it should be:
an encoder in the drm/meson core driver.

The bridge functions are copied as-is, except:
- the encoder init uses the simple kms helper
- the mode_set has been moved to atomic_enable()
- debug prints are converted to dev_debg()

For now the bridge attach flags is 0, DRM_BRIDGE_ATTACH_NO_CONNECTOR
will be handled later.

The meson dw-hdmi glue is slightly fixed to live without the
encoder in the same driver.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
[narmstrong: fixed warning because missing meson_encoder_hdmi.h include]
Link: https://patchwork.freedesktop.org/patch/msgid/20211020123947.2585572-4-narmstrong@baylibre.com