drm/amdgpu: deduplicate ring preempt ib function

drm/amdgpu: deduplicate ring preempt ib function

The ring preemption function is identical for both gfx_v11_0 and
gfx_v12_0. This patch refactors the code by moving the core logic
into a generic function inside amdgpu_gfx.c to reduce code
duplication and simplify future maintenance.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Leonardo Cesar <leonardocesar@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>