drm/amdgpu: Surface svm_default_granularity, a RW module parameter

Enables users to update SVM's default granularity, used in
buffer migration and handling of recoverable page faults.
Param value is set in terms of log(numPages(buffer)),
e.g. 9 for a 2 MIB buffer

Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>