Return 0 for success scenairos in 'gmc_v6/7/8/9_0_hw_init()'

Fixes the below:
drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c:920 gmc_v6_0_hw_init() warn: missing error code? 'r'
drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c:1104 gmc_v7_0_hw_init() warn: missing error code? 'r'
drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c:1224 gmc_v8_0_hw_init() warn: missing error code? 'r'
drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c:2347 gmc_v9_0_hw_init() warn: missing error code? 'r'

Fixes: fac4ebd7 ("drm/amdgpu: Fix with right return code '-EIO' in 'amdgpu_gmc_vram_checking()'")
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>