drm/amd/display: Add option to retrieve detile buffer size

[WHY]
For better power profiling knowing the detile
buffer size at a given point in time
would be useful.

[HOW]
Add interface to retrieve detile buffer from
dc state.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Sung Lee <Sung.Lee@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>