drm/amdgpu: Fix SDMA UTC_L1 handling during start/stop sequences

drm/amdgpu: Fix SDMA UTC_L1 handling during start/stop sequences

This commit makes two key fixes to SDMA v4.4.2 handling:

1. disable UTC_L1 in sdma_cntl register when stopping SDMA engines
   by reading the current value before modifying UTC_L1_ENABLE bit.

2. Ensure UTC_L1_ENABLE is consistently managed by:
   - Adding the missing register write when enabling UTC_L1 during start
   - Keeping UTC_L1 enabled by default as per hardware requirements

v2: Correct SDMA_CNTL setting (Philip)

Suggested-by: Jonathan Kim <jonathan.kim@amd.com>
Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>