drm/amd/display: fix the white screen issue when >= 64GB DRAM

Dropping bit 31:4 of page table base is wrong, it makes page table
base points to wrong address if phys addr is beyond 64GB; dropping
page_table_start/end bit 31:4 is unnecessary since dcn20_vmid_setup
will do that. Also, while we are at it, cleanup the assignments using
upper_32_bits()/lower_32_bits() and AMDGPU_GPU_PAGE_SHIFT.

Cc: stable@vger.kernel.org
Link: drm/amd#2354


Fixes: 81d0bcf9 ("drm/amdgpu: make display pinning more flexible (v2)")
Acked-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Co-developed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>