drm/amd/display: Make observers const-correct

drm/amd/display: Make observers const-correct

[Why]
Observers which do not modify their pointer arguments should take
them as const. This clearly signals their intent to the caller,
making it clear that the function is safe to call multiple times,
or remove the call if the result is no longer necessary.

[How]
Made const-correct all of the functions below:
* full_update_required[_weak]
* fast_updates_exist
* fast_update_only
* dc_can_clear_cursor_limit
* dc_stream_get_status (added const named overload)

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Dominik Kaszewski <dominik.kaszewski@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>