selftests/drivers/gpu: Add error messages to drm_mm.sh

Add error messages when the module test-drm_mm is not found or could
not be removed to make tests output more readable.

Signed-off-by: Gautam Menghani <gautammenghani201@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>