drm: introduce DRM_CAP_ATOMIC_ASYNC_PAGE_FLIP

This new kernel capability indicates whether async page-flips are
supported via the atomic uAPI. DRM clients can use it to check
for support before feeding DRM_MODE_PAGE_FLIP_ASYNC to the kernel.

Make it clear that DRM_CAP_ASYNC_PAGE_FLIP is for legacy uAPI only.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20231122161941.320564-4-andrealmeid@igalia.com