drm/amd/display: move location of dmub_srv.h file

[Why]
Make a separation of what belongs in the differen dmub
headers

dmub_srv.h is for exposing dmub srv interface to rest of
driver.

other headers inside dmub/inc exposes cmds and definitions
that are owned by the firmware

[How]
keep firmware owned definitions in dmub/inc

move stuff that is purely driver interface headers to dmub/
since those are interface calls that are defined for rest of
driver to use

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>