drm/i915/pps: abstract intel_pps_encoder_reset()

Add an "encoder reset" call to hide some more pps functions, and clean
up the callers. A minor functional change is not holding the pps lock
across the whole operation in intel_dp_encoder_reset, but instead doing
it in two steps.

v2: rename intel_pps_reinit to intel_pps_encoder_reset for clarity

Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/84a50f2700b19c6719cd3e1e931c64f1e2027551.1610127741.git.jani.nikula@intel.com