drm: crtc_helper: Update hwmode before mode_set call

Just as we provide crtc->mode pre-populated with the requested mode,
move adjusted_mode into hwmode before we call the crtc's mode_set,
making sure to restore it on failure.

Allows drivers which thoughtlessly discard adjusted_mode in their
mode_set hooks (e.g. Exynos) to use hwmode directly, and also provides
some neat symmetry with crtc->mode.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>