drm/i915: Move encoder suspend/shutdown helpers to intel_encoder.c

Move the encoder suspend/shutdown helpers to intel_encoder.c, this being
the logical place for encoder functions.

This also allows sharing the above helpers with the xe driver, done in a
follow-up patch.

While at it rename the functions using the usual intel_encoder prefix
and in the functions rename the dev_priv parameter to i915.

v2: Remove extra w/s in the include section. (Jani)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> # v1
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240618125255.4080303-1-imre.deak@intel.com