directfb: Really disable mesa

Mesa was disabled in the .inc but not in the main recipe where a separate
EXTRA_OECONF is used. Fix disabling mesa there too to avoid build failures.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
sanity.bbclass: Check for DISPLAY for qemutests in BB_ORIGENV now

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>