drm/amdgpu: Mark mmhub_v1_8_mmea_err_status_reg as __maybe_unused

Silencing the compiler from below compilation error:

drivers/gpu/drm/amd/amdgpu/mmhub_v1_8.c:704:23: error: variable 'mmhub_v1_8_mmea_err_status_reg' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
static const uint32_t mmhub_v1_8_mmea_err_status_reg[] = {
                      ^
1 error generated.

Mark the variable as __maybe_unused to make it clear to clang that this
is expected, so there is no more warning.

Cc: Christian König <christian.koenig@amd.com>
Cc: Lijo Lazar <lijo.lazar@amd.com>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>