drm/fourcc: document modifier uniqueness requirements

 * may preserve meaning - such as number of planes - from the fourcc code,

 * whereas others may not.

 *

 * Modifiers must uniquely encode buffer layout. In other words, a buffer must

 * match only a single modifier. A modifier must not be a subset of layouts of

 * another modifier. For instance, it's incorrect to encode pitch alignment in

 * a modifier: a buffer may match a 64-pixel aligned modifier and a 32-pixel

 * aligned modifier. That said, modifiers can have implicit minimal

 * requirements.

 *

 * For modifiers where the combination of fourcc code and modifier can alias,

 * a canonical pair needs to be defined and used by all drivers. Preferred

 * combinations are also encouraged where all combinations might lead to

 * confusion and unnecessarily reduced interoperability. An example for the

 * latter is AFBC, where the ABGR layouts are preferred over ARGB layouts.

 *

 * There are two kinds of modifier users:

 *

 * - Kernel and user-space drivers: for drivers it's important that modifiers

 *   don't alias, otherwise two drivers might support the same format but use

 *   different aliases, preventing them from sharing buffers in an efficient

 *   format.

 * - Higher-level programs interfacing with KMS/GBM/EGL/Vulkan/etc: these users

 *   see modifiers as opaque tokens they can check for equality and intersect.

 *   These users musn't need to know to reason about the modifier value

 *   (i.e. they are not expected to extract information out of the modifier).

 *

 * Vendors should document their modifier usage in as much detail as

 * possible, to ensure maximum compatibility across devices, drivers and

 * applications.