drm/amdgpu: Replace ternary operator with min() in 'amdgpu_iomem_read'

Fixes the following coccicheck:

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c:2427:16-17: WARNING opportunity for min()

min() macro is defined in include/linux/minmax.h. It avoids multiple
evaluations of the arguments when non-constant and performs strict
type-checking.

Cc: Guchun Chen <guchun.chen@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>