drm/amd/display: fix issue with DC brightness low with VB

[Why]
The problem is that we accidentally stopped loading some of the IRAM bytes
used for the backlight ramping mechanism. This happened when we
started reserving some region of IRAM as DMCU FW write only.

[How]
This change will define a start+end region for the IRAM read only region.
So the parameters needed for the backlight operation will be loaded
since it will be defined outside of the read only region.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>