drm/i915/display: move needs_modeset to an inline in header

This function is going to be used in a later change, so clean it
up first before moving it.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201221090449.8288-1-jani.nikula@intel.com