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

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

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Jonathan Hunter <jonathanh@nvidia.com>
Cc: linux-tegra@vger.kernel.org
[mlankhorst: Keep tegra_crtc_reset at the same place and predeclare
 tegra_crtc_atomic_destroy_state (tagr)]
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-15-maarten.lankhorst@linux.intel.com