drm/meson: Fix atomic mode switching regression

Since commit 2bcd3eca when switching mode from X11 (ubuntu mate for
example) the display gets blurry, looking like an invalid framebuffer width.

This commit fixed atomic crtc modesetting in a totally wrong way and
introduced a local unnecessary ->enabled crtc state.

This commit reverts the crctc _begin() and _enable() changes and simply
adds drm_atomic_helper_commit_tail_rpm as helper.

Reported-by: Tony McKahan <tonymckahan@gmail.com>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Fixes: 2bcd3eca ("drm/meson: Fixes for drm_crtc_vblank_on/off support")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[narmstrong: fixed blank line issue from checkpatch]
Link: https://patchwork.freedesktop.org/patch/msgid/20190114153118.8024-1-narmstrong@baylibre.com