vgaarb: remove bogus checks

neither ->release() nor ->poll() can be called unless ->open()
has succeeded on the same struct file, so checking for "has
open() failed" is pointless.  What's more, ->poll() doesn't
return -E... - it always returns a bitmap of POLL... values,
so the dead code in that one had been actively bogus.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>