drm/amd: Split up UVD suspend into prepare and suspend steps

amdgpu_uvd_suspend() allocates memory and copies objects into that
allocated memory.  This fails under memory pressure.  Instead move
majority of this code into a prepare step when swap can still be
allocated.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>