drm/amdgpu: Avoid possiblity of kernel crash in 'gmc_v8_0, gmc_v7_0_init_microcode()'

If the function 'gmc_v8_0_ or gmc_v7_0_init_microcode()' fails, the
driver will just fail to load, hence return -EINVAL rather having BUG(),
fixes WARNING: Do not crash the kernel unless it is absolutely
unavoidable--use WARN_ON_ONCE() plus recovery code (if feasible) instead
of BUG() or variants

Fixes: 2f77b593 ("drm/amdgpu: Fix error & warnings in gmc_v8_0.c")
Fixes: 0cfc1d68 ("drm/amdgpu: Fix errors & warnings in gmc_ v6_0, v7_0.c")
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
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: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>