drm/amdgpu: add helper function to query psp runtime db entry (v2)

PSP will dump various boot up information into a
portion of local frame buffer, called runtime database.
The helper function is used for driver to query those
shared information.

v2: init ret and check !ret to exit loop as soon as
found the entry

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: John Clements <john.clements@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>