mesa: switch mesa to swrast renderer for mx8mm machine, resolve build issue
After mesa update to 19.1.0, the OSMesa is not anymore explicitly set to
'classic' rather than defaults to 'none'. If the DRI driver is included
in the build and neither gl nor 'classic' osmesa is used - this causes
the build to fail.
This commit disables the gallium for mx8mm, sets the dri drivers to
swrast and explicitly defines the 'classic' osmesa to be built.
gallium is disabled since current support state of i.MX8M Mini GPU in
gallium is unknown.
