drm/amdgpu: remove set but not used variable 'mc_shared_chmap' from 'gfx_v6_0.c' and 'gfx_v7_0.c'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/gfx_v6_0.c: In function
‘gfx_v6_0_constants_init’:
drivers/gpu/drm/amd/amdgpu/gfx_v6_0.c:1579:6: warning: variable
‘mc_shared_chmap’ set but not used [-Wunused-but-set-variable]

drivers/gpu/drm/amd/amdgpu/gfx_v7_0.c: In function
‘gfx_v7_0_gpu_early_init’:
drivers/gpu/drm/amd/amdgpu/gfx_v7_0.c:4262:6: warning: variable
‘mc_shared_chmap’ set but not used [-Wunused-but-set-variable]

Fixes: 2cd46ad2 ("drm/amdgpu: add graphic pipeline implementation for si v8")
Fixes: d93f3ca7 ("drm/amdgpu/gfx7: rework gpu_init()")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>