nv50_disp_chan_mthd: ensure mthd is not NULL

Dear Ben Skeggs,

Please find attached a patch solving a blocking issue I encountered: https://bugzilla.kernel.org/show_bug.cgi?id=206299

Basically, running at least a RTX2080TI on Xen makes a bad mmio error which causes having 'mthd' pointer to be NULL in 'channv50.c'. From the code, it's assumed to be not NULL by accessing directly 'mthd->data[0]' which is the reason of the kernel panic. I simply check if the pointer is not NULL before continuing.

Best regards,

Frédéric Pierret