intel: introduce to_bo_gem() helper

...to minimise misuse of bo_gem.
If the variable is declared at the top of the function and then used
for two (or more) different contexts this can cause confusion and errors.

Just introduce a wrapper, which can be used in a once off situations.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by Chris Wilson <chris@chris-wilson.co.uk>