drm/fourcc: fix AMD modifiers PACKERS field doc

This field doesn't alias with BANK_XOR_BITS: PACKERS is bits 27:29 while
BANK_XOR_BITS is bits 24:26.

Fixes: 8ba16d59 ("drm/fourcc: Add AMD DRM modifiers.")
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>