drm/amdgpu/gmc6: fix DMA mask from 44 to 40 bits

The DMA mask on SI parts is 40 bits not 44.  Copy
paste typo.

Fixes: 244511f3 ("drm/amdgpu: simplify and cleanup setting the dma mask")
Bug: drm/amd#1762


Acked-by: Christian König <christian.koenig@amd.com>
Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>