drm/amdgpu: Fix the vram base start address

If the size returned by drm buddy allocator is higher than
the required size, we take the higher size to calculate
the buffer start address. This is required if we couldn't
trim the buffer to the requested size. This will fix the
display corruption issue on APU's which has limited VRAM
size.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2859
Fixes: 0a1844bf

 ("drm/buddy: Improve contiguous memory allocation")
Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>