drm/amd: append missing includes

This fixes warnings caused by global functions lacking prototypes:, such as:

 warning: no previous prototype for 'dcn303_hw_sequencer_construct'
 [-Wmissing-prototypes]
 12 | void dcn303_hw_sequencer_construct(struct dc *dc)
    |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ...
 warning: no previous prototype for ‘amdgpu_has_atpx’
 [-Wmissing-prototypes]
 76 | bool amdgpu_has_atpx(void) {
    |      ^~~~~~~~~~~~~~~

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>