drm/amd/display: rename hdmi_frl_pcon_support

hdmi_frl_pcon_support has been the source of confusion. So, rename it to
dp_hdmi21_pcon_support.

Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>