drm/amdgpu: Replace DEFINE_SIMPLE_ATTRIBUTE with DEFINE_DEBUGFS_ATTRIBUTE

Fix the following coccicheck warning:
./drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c:1589:0-23: WARNING:
fops_ib_preempt should be defined with DEFINE_DEBUGFS_ATTRIBUTE
./drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c:1592:0-23: WARNING:
fops_sclk_set should be defined with DEFINE_DEBUGFS_ATTRIBUTE

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>