drm/amd/display: avoid 64-bit division

On 32-bit architectures, dividing a 64-bit integer in the kernel
leads to a link error:

ERROR: "__udivdi3" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: "__divdi3" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!

Change the two recently introduced instances to a multiply+shift
operation that is also much cheaper on 32-bit architectures.
We can do that here, since both of them are really 32-bit numbers
that change a few percent.

Fixes: bedbbe6a ("drm/amd/display: Move link functions from dc to dc_link")
Fixes: f18bc4e5

 ("drm/amd/display: update calculated bounding box logic for NV")
Acked-by: Slava Abramov <slava.abramov@amd.com>
Tested-by: Slava Abramov <slava.abramov@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>