drm/radeon: change vblank_time's calculation method to reduce computational error.

[ Upstream commit 02cfb5fccb0f9f968f0e208d89d9769aa16267bc ]

Ported from Rex's amdgpu change.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>