gbm: Export a per plane getter for offset

Unlike stride, there was no previous offset getter, so it can be right
on the first try.

v2: Return EINVAL when plane is greater than total planes to make it
match the similar APIs.
Avoid leak after fromPlanar (Daniel)
Make sure when getting offsets we consider dumb images (Daniel)

v3: Use Jason's recommendation for handling the non-planar case.

v4: Return int64_t so we can get real errors

v5: Add an assertion for dumb BOs (Jason)

Signed-off-by: Ben Widawsky <[email protected]>
Reviewed-by: Eric Engestrom <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>
Acked-by: Daniel Stone <[email protected]>

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