drm/amd/display: correct kernel-doc in dml21_wrapper.h

drm/amd/display: correct kernel-doc in dml21_wrapper.h

Fix all kernel-doc warnings in dml21_wrapper.h:
- add missing @dml_ctx entries (2 places)
- fix function prototype typo for dml21_create()
- change a blank kernel-doc line to " *"

Fixes these warnings:
Warning: drivers/gpu/drm/amd/display/dc/dml2_0/dml21/dml21_wrapper.h:30
 function parameter 'dml_ctx' not described in 'dml21_create'
Warning: drivers/gpu/drm/amd/display/dc/dml2_0/dml21/dml21_wrapper.h:30
 expecting prototype for dml2_create(). Prototype was for dml21_create()
 instead
Warning: drivers/gpu/drm/amd/display/dc/dml2_0/dml21/dml21_wrapper.h:55
 bad line:
Warning: drivers/gpu/drm/amd/display/dc/dml2_0/dml21/dml21_wrapper.h:61
 function parameter 'dml_ctx' not described in 'dml21_validate'

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