drm/amdgpu: remove set but not used variable 'mc_shared_chmap'

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

drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c: In function
‘gfx_v8_0_gpu_early_init’:
drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c:1713:6: warning: variable
‘mc_shared_chmap’ set but not used [-Wunused-but-set-variable]

Fixes: 0bde3a95 ("drm/amdgpu: split gfx8 gpu init into sw and hw parts")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>