tests/amdgpu: divide dispatch test into compute and gfx

for better clarification

v2: accordingly change dispatch_test caller in gpu_reset test

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Flora Cui <flora.cui@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>