Android: clean-up and fix DRI module path handling

Android: clean-up and fix DRI module path handling

MESA_DRI_MODULE_PATH is only getting set for classic DRI drivers and may or
may not be set correctly for gallium_dri.so depending on the makefile
include ordering. For Android 6 and earlier it is fine, but with build
system changes in AOSP master, it is not.

Move the path variables to a single place at the top level and introduce
MESA_DRI_MODULE_REL_PATH for Android 5 and later which require relative
paths. With this, there is a single variable to change.

Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>