drm/i915: Stop frobbing crtc->base.mode

The core no longer uses drm_crtc_state::mode with atomic drivers,
so let's stop frobbing it in the driver. For the user mode readout
we'll just use an on stack mode.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191029145526.10308-1-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>