drm/amd/display: refine code for dmcub inbox1 ring buffer debug

[Why]
1. To watch dmcub inbox1 ring buffer cmd type without tools
2. dmub_cmd_PLAT_54186_wa 66 bytes

[How]
Added dmcub cmd type enum: unsigned char for debug use only,
also fixed 66 bytes issue by using unsigned int in bit
define instead of unsigned char.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Fudongwang <fudong.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>