[Nouveau] nv50_disp_chan_mthd: ensure mthd is not NULL

[Frédéric Pierret]

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

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