drm/gem: Ignore color planes that are unused by framebuffer format

Only handle color planes that exist in a framebuffer's color format.
Ignore non-existing planes.

So far, several helpers assumed that all 4 planes are available and
silently ignored non-existing planes. This lead to subtil bugs with
uninitialized data in instances of struct iosys_map. [1]

Signed-off-by: Thomas Zimmermann <[email protected]>
Reviewed-by: Javier Martinez Canillas <[email protected]>
Tested-by: Noralf Trønnes <[email protected]>
Acked-by: Christian König <[email protected]>
Link: https://lore.kernel.org/dri-devel/[email protected]/T/#md0172b10bb588d8f20f4f456e304f08d2a4505f7 # 1
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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