drm/mali: Convert to using __drm_atomic_helper_crtc_reset() for reset.

Convert mali to using __drm_atomic_helper_crtc_reset(), instead of
writing its own version. Instead of open coding
malidp_crtc_destroy_state(), call it directly for freeing the old state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Brian Starkey <brian.starkey@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-6-maarten.lankhorst@linux.intel.com