drm/amd/amdgpu: expose fragment size as module parameter (v2)

Allow overrides on the command line.

v2: agd: sqaush in spelling fix and bogus default value warning

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