codec/vout: prevent gles vout display with x420/CVPX_P010 on darwin mobile hosts

Given it's properly supported by darwin desktop hosts, x420 (or VLC_CODEC_CVPX_P010) are chroma formats only supported by Metal backends on darwin mobile hosts.
The current implementation in videotoolbox prevents the use of this chroma at compile time in decoder configuration, hence preventing the use of another display backends if we provide another way to support it on mobile.
With this change, given the chroma format is VLC_CODEC_CVPX_P010, the gles2 vout display will fail on darwin mobile hosts, giving the opportunity to fallback to another vout display that may be able to support it, like Apple's AVSampleBufferDisplayLayer renderer.