drm/amdgpu: fix zero-size GDS range init on RDNA4

Summary

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu: fix zero-size GDS range init on RDNA4 RDNA4 (GFX 12) hardware removes the GDS, GWS, and OA on-chip memory resources. The gfx_v12_0 initialisation code correctly leaves adev->gds.gds_size, adev->gds.gws_size, and adev->gds.oa_size at zero to reflect this. amdgpu_ttm_init() unconditionally calls amdgpu_ttm_init_on_chip() for each of these resources regardless of size. When the size is zero, amdgpu_ttm_init_on_chip() forwards the call to ttm_range_man_init(), which calls drm_mm_init(mm, 0, 0). drm_mm_init() immediately fires DRM_MM_BUG_ON(start + size <= start) -- trivially true when size is zero -- crashing the kernel during modprobe of amdgpu on an RX 9070 XT. Guard against this by returning 0 early from amdgpu_ttm_init_on_chip() when size_in_page is zero. This skips TTM resource manager registration for hardware resources that are absent, without affecting any other GPU type. DRM_MM_BUG_ON() only asserts if CONFIG_DRM_DEBUG_MM is enabled in the kernel config. This is apparently rarely enabled as these chips have been in the market for over a year and this issue was only reported now. Oops-Analysis: http://oops.fenrus.org/reports/bugzilla.korg/221376/report.html (cherry picked from commit 5719ce5865279cad4fd5f01011fe037168503f2d)