drm/amd/display: Indirect reg read macro with shift and mask

[Why]
Recent double buffering changes for dcn2 use IX_REG_READ.
However, this macro returns the full register value, with the need to
manually shift and mask it to retrieve field data.

[How]
Create new IX_REG_GET macro that handles shift and mask.
Use this for double buffering reads instead of IX_REG_READ.

Signed-off-by: Noah Abradjian <noah.abradjian@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>