drm/amd/amdgpu: introduce DRM_AMDGPU_WERROR

We want to do -Werror builds on our CI. However, non-amdgpu breakages
have prevented us from doing so thus far. Also, there are a number of
additional checks that we should enable, that the community cares about
and are hidden behind -Wextra. So, define DRM_AMDGPU_WERROR to only
enable -Werror for the amdgpu kernel module and enable -Wextra while
disabling all of the checks that are too noisy.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Kenny Ho <kenny.ho@amd.com>
Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Kenny Ho <Kenny.Ho@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>