gamescopectl: Add HDR info to backend_info

[flibitijibibo]

Feature request

Currently the backend_info dump is shared across all backends - it would be nice to get HDR-specific information for the DRM backend, for example this block here:

gamescope/src/Backends/DRMBackend.cpp

Lines 2237 to 2241 in d317492

	m_Mutable.HDR.bExposeHDRSupport = otHDRInfo->get_or( "supported", false );
	m_Mutable.HDR.eOutputEncodingEOTF = otHDRInfo->get_or( "eotf", EOTF_Gamma22 );
	m_Mutable.HDR.uMaxContentLightLevel = nits_to_u16( otHDRInfo->get_or( "max_content_light_level", 400.0f ) );
	m_Mutable.HDR.uMaxFrameAverageLuminance = nits_to_u16( otHDRInfo->get_or( "max_frame_average_luminance", 400.0f ) );
	m_Mutable.HDR.uMinContentLightLevel = nits_to_u16_dark( otHDRInfo->get_or( "min_content_light_level", 0.1f ) );

Anything that could help debug ValveSoftware/SteamOS#1384 will be very useful!