drm/amdgpu: keep vga memory on MacBooks with switchable graphics

On Intel MacBookPros with switchable graphics, when the iGPU
is enabled, the address of VRAM gets put at 0 in the dGPU's
virtual address space.  This is non-standard and seems to cause
issues with the cursor if it ends up at 0.  We have the framework
to reserve memory at 0 in the address space, so enable it here if
the vram start address is 0.

Reviewed-and-tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4302
Cc: stable@vger.kernel.org
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: alexdeucher

drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c

@@ -1068,6 +1068,16 @@ void amdgpu_gmc_get_vbios_allocations(struct amdgpu_device *adev)
 	case CHIP_RENOIR:
 		adev->mman.keep_stolen_vga_memory = true;
 		break;
+	case CHIP_POLARIS10:
+	case CHIP_POLARIS11:
+	case CHIP_POLARIS12:
+		/* MacBookPros with switchable graphics put VRAM at 0 when
+		 * the iGPU is enabled which results in cursor issues if
+		 * the cursor ends up at 0.  Reserve vram at 0 in that case.
+		 */
+		if (adev->gmc.vram_start == 0)
+			adev->mman.keep_stolen_vga_memory = true;
+		break;
 	default:
 		adev->mman.keep_stolen_vga_memory = false;
 		break;