drm/amdgpu: add "restore" missing variable comment

add "restore" missing variable in the fucntions
sdma_v4_4_2_page_resume and sdma_v4_4_2_inst_start.

This fixes the warning:
warning: Function parameter or struct member 'restore' not described in 'sdma_v4_4_2_page_resume'
warning: Function parameter or struct member 'restore' not described in 'sdma_v4_4_2_inst_start'

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>