drm/i915: Clarify logic for initial modeset

Currently we both set mode->private_flags to some value and also use
the pipe_config quirk. But since the pipe_config quirk isn't tied to
the lifetime of the mode object we need to check both.

Simplify this by only using mode.private_flags and stop using the
INHERITED_MODE quirk. Also for clarity add an explicit #define for
that driver priavete mode flag.

By using crtc_state->mode_changed we can also remove the recalc local
variable.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>