drm: move tinydrm format conversion helpers to new drm_format_helper.c

Also rename them from tinydrm_* to drm_fb_*
Pure code motion, no functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190405095219.9231-2-kraxel@redhat.com