freedreno: android: fix build failure on android due to python version

freedreno: android: fix build failure on android due to python version

The freedreno gen_header.py script now only works under python3.
It contains a "print()" call which prints a blank line under python3
but prints "()" under python2.7.

However the Android build currently uses python2.

This leads to incorrect code generation and a later build error.

.../STATIC_LIBRARIES/libfreedreno_registers_intermediates/registers/adreno_common.xml.h:163:2: error: expected identifier or '('
()

Fix this by adding MESA_PYTHON3 and using it for the freedreno scripts.

Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3736>