drm/amd/display: fix function scopes

This turns previously global functions into static, thus removing
compile-time warnings such as:

warning: no previous prototype for 'get_highest_allowed_voltage_level'
[-Wmissing-prototypes]
742 | unsigned int get_highest_allowed_voltage_level(uint32_t chip_family, uint32_t hw_internal_rev, uint32_t pci_revision_id)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
warning: no previous prototype for 'rv1_vbios_smu_send_msg_with_param'
[-Wmissing-prototypes]
102 | int rv1_vbios_smu_send_msg_with_param(struct clk_mgr_internal *clk_mgr, unsigned int msg_id, unsigned int param)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Changes since v1:
- As suggested by Rodrigo Siqueira:
1. Rewrite function signatures to make them more readable.
2. Get rid of unused functions in order to remove 'defined but not
used' warnings.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Isabella Basso <isabbasso@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drm/amd/display: add cyan_skillfish display support

[Why]
add display related cyan_skillfish files in.

makefile controlled by CONFIG_DRM_AMD_DC_DCN201 flag.

v2: squash in clang fixes from Harry, Nathan
v3: squash in missing CONFIG_DRM_AMD_DC check (Alex)

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Jun Lei <jun.lei@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>