drm/amd/display: Fix unused variable ‘should_lock_all_pipes’

Fix below compilation error:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:3524:7: error: unused variable 'should_lock_all_pipes' [-Werror,-Wunused-variable]
        bool should_lock_all_pipes = (update_type != UPDATE_TYPE_FAST);

Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>