[PATCH] drm/amd/display: Fix AMDGPU_MAX_BL_LEVEL value

[Marcus Seyfarth]

Just FYI, using Clang-21git, I see this new warning with the current patch:

drivers/gpu/drm/amd/amdgpu/atombios_encoders.c:131:11: warning:
implicit conversion from 'int' to 'u8' (aka 'unsigned char') changes
value from 65535 to 255 [-Wconstant-conversion]
 131 |                 level = AMDGPU_MAX_BL_LEVEL;
     |                       ~ ^~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/amdgpu_mode.h:438:29: note: expanded from
macro 'AMDGPU_MAX_BL_LEVEL'
 438 | #define AMDGPU_MAX_BL_LEVEL 0xFFFF
     |                             ^~~~~~
1 warning generated.