vl/dri3: get Makefile properly

vl/dri3: get Makefile properly

From original commit, the macro "if HAVE_DRI3" was in Makefile.sources,
this file is shared with SCons, SCons is not able to parse this marco,
the SCons build failed. Jose quickly gave two approaches and quick fix
with his second approach, thanks Jose for the solutions and fixes.

This patch is Jose's first approach, and it's more proper, because the
dri3 c file should not be included to build when DRI3 is not enabled.

Signed-off-by: Leo Liu <leo.liu@amd.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>