drm/amd: Remove unused declarations

amdgpu_gart_table_vram_pin() and amdgpu_gart_table_vram_unpin() has
been removed since commit 575e55ee ("drm/amdgpu: recover gart table
at resume") remain the declarations untouched in the header files.

Besides, amdgpu_dm_display_resume() has also beed removed since
commit a80aa93d ("drm/amd/display: Unify dm resume sequence into a
single call"). So, let's remove this unused declarations.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>