drm/amdgpu: fix compiler warnings

Fixes below compiler warnings:
 CC [M]  drivers/gpu/drm/amd/amdgpu/amdgpu_device.o
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:381:1: warning: ‘static’ is not at beginning of declaration [-Wold-style-declaration]
  381 | void static inline amdgpu_mm_wreg_mmio(struct amdgpu_device *adev, uint32_t reg, uint32_t v, uint32_t acc_flags)
      | ^~~~
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:381:1: warning: ‘inline’ is not at beginning of declaration [-Wold-style-declaration]
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c: In function ‘amdgpu_device_fini’:
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:3381:6: warning: variable ‘r’ set but not used [-Wunused-but-set-variable]
 3381 |  int r;
      |      ^

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>