media: venus: fix compile-test build on non-qcom ARM platform

If QCOM_MDT_LOADER is enabled, but ARCH_QCOM is not, we run into
a build error:

ERROR: "qcom_mdt_load" [drivers/media/platform/qcom/venus/venus-core.ko] undefined!
ERROR: "qcom_mdt_get_size" [drivers/media/platform/qcom/venus/venus-core.ko] undefined!

This changes the 'select' statement again, so we only try to enable
those symbols when the drivers will actually get built, and explicitly
test for QCOM_MDT_LOADER to be enabled before calling into it.

Fixes: 76724b30 ("[media] media: venus: enable building with COMPILE_TEST")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>