drm: drm_fourcc: Add generic alias for 16_16_TILE modifier

In cases such as DRM_FORMAT_MOD_SAMSUNG_16_16_TILE, the modifier
describes a generic pixel re-ordering which can be applicable to
multiple vendors.

Define an alias: DRM_FORMAT_MOD_GENERIC_16_16_TILE, which can be
used to describe this layout in a vendor-neutral way, and add a
comment about the expected usage of such "generic" modifiers.

Changes in v2:
 - Move note about future cases to comment (Daniel)

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200626164800.11595-1-brian.starkey@arm.com