drm/radeon: drop setting UPLL to sleep mode

Just keep it working, seems to fix some PLL problems.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=73378

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org