drm: Check actual format for legacy pageflip.

With modifiers one can actually have different format_info structs
for the same format, which now matters for AMDGPU since we convert
implicit modifiers to explicit modifiers with multiple planes.

I checked other drivers and it doesn't look like they end up triggering
this case so I think this is safe to relax.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Fixes: 816853f9 ("drm/amd/display: Set new format info for converted metadata.")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210110021142.28221-1-bas@basnieuwenhuizen.nl