drm/i915: Use the new __drm_atomic_helper_crtc_reset() helper.

i915 has its own hw readout and doesn't use the reset helpers directly.
Still it has 2 places where it initialises the crtc_state. Fix those
by calling __drm_atomic_helper_crtc_reset().

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Not-nacked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-8-maarten.lankhorst@linux.intel.com