drm/amd/amdgpu: Remove double lock from gfx v6

The function gfx_v6_0_get_cu_info() was taking the
grbm_idx_mutex which was then taken by a dependent
function gfx_v6_0_get_cu_active_bitmap().

This patch removes the select from the parent function
to avoid the double lock.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>