Daniel Stone authored Apr 28, 2022 and Pekka Paalanen committed May 20, 2022

'depth' isn't actually used to determine the bit depth of meaningful
components generally, but specifically to determine whether we can use
the legacy drmModeAddFB (pre-AddFB2) with those formats.

Rename the member to make it more clear what it's used for.

Signed-off-by: Daniel Stone <daniels@collabora.com>