drm/amdgpu: refine default gart size

Be more explicit and add comments explaining each case.
Also s/gart/GART/ in the parameter string as per Felix'
suggestion.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>