drm/amd/display: dmub_cmd.h: add missing kernel-doc for enums

drm/amd/display: dmub_cmd.h: add missing kernel-doc for enums

For enums that have a kernel-doc ("/**") comment block, add or correct
their kernel-doc to eliminate all kernel-doc enum warnings.

Some of these needed "struct" changed to "enum" in the kernel-doc header.
For a few of them, I changed the "/**" comment to a plain "/*"
comment since there was no kernel-doc content there.

Example:

Warning: drivers/gpu/drm/amd/display/dmub/inc/dmub_cmd.h:2673 Enum value 'FAMS2_ALLOW_DELAY_CHECK_NONE' not described in enum 'dmub_fams2_allow_delay_check_mode'
Warning: drivers/gpu/drm/amd/display/dmub/inc/dmub_cmd.h:2673 Enum value 'FAMS2_ALLOW_DELAY_CHECK_FROM_START' not described in enum 'dmub_fams2_allow_delay_check_mode'
Warning: drivers/gpu/drm/amd/display/dmub/inc/dmub_cmd.h:2673 Enum value 'FAMS2_ALLOW_DELAY_CHECK_FROM_PREPARE' not described in enum 'dmub_fams2_allow_delay_check_mode'

All .o files are the same with or without this patch.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>