gallium/radeon: implement randomized SDMA texture copy testing (v2)

gallium/radeon: implement randomized SDMA texture copy testing (v2)

v2: - adjustments for exercising all important SDMA code paths
    - decrease the probability of getting huge sizes (faster testing)
    - increase the probability of getting power-of-two dimensions
    - change the memory cap to 128MB (faster testing)
    - better detect which engine has been used

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>