drm/i915/display: split out dpt out of intel_display.c

Let's try to reduce the size of intel_display.c, not increase it.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/934a2a0db05e835f6843befef6082e2034f23b3a.1629721467.git.jani.nikula@intel.com