drm/amdgpu: Add description for AMD_IP_BLOCK_TYPE_VPE

AMD_IP_BLOCK_TYPE_VPE is a new IP BLOCK type for Video Processing Engine,
but currently lacks description.

Fix the documentation warning:
warning: Enum value 'AMD_IP_BLOCK_TYPE_VPE' not described in
enum 'amd_ip_block_type'

Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Reviewed-by: Lang Yu <lang.yu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>