drm: Fix example comment of format modifier blob

To represent formats 98-102, the supported formats mask must be
0x7c00000000 and not 0x3c00000000.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170831195215.13302-1-krisman@collabora.co.uk