amdgpu/dc: remove various variables that are defined but never used

There are several variables that are defined and never used and hence can
be removed. Remove them. Cleans up clang -Wunused-const-variable warnings:

warning: ‘dvi_hdmi_dongle_signature_str’ defined but not used
warning: ‘dce11_one_lpt_channel_max_resolution’ defined but not used
warning: ‘ddc_hw_status_addr’ defined but not used

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>