drm/ast: fix missing break in switch statement for format->cpp[0] case 4

Currently the switch statement for format->cpp[0] value 4 assigns
color_index which is never read again and then falls through to the
default case and returns. This looks like a missing break statement
bug. Fix this by adding a break statement.

Addresses-Coverity: ("Unused value")
Fixes: 259d14a7 ("drm/ast: Split ast_set_vbios_mode_info()")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200610115804.1132338-1-colin.king@canonical.com