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>