drm/amdgpu: fix number of fence calculations

Since adding gang submit we need to take the gang size into account
while reserving fences.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 4624459c ("drm/amdgpu: add gang submit frontend v6")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>