drm/ttm: Simplify ttm_eu_reserve_buffers()

Hoist the comparison of the ret to -EDEADLK above
the two code paths to simplify the function.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>