drm/i915/display: move modeset probe/remove functions to intel_display_driver.c

High level display functionality only called from driver top level code.

v2:
- Remove unnecessary declarations for intel_sanitize_watermarks() and
  intel_atomic_check() (Gustavo)

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c00df5cfc233520bc28dd972296197a8a619a6a0.1681465222.git.jani.nikula@intel.com