drm/i915: Multi-BB execbuf

Allow multiple batch buffers to be submitted in a single execbuf IOCTL
after a context has been configured with the 'set_parallel' extension.
The number batches is implicit based on the contexts configuration.

This is implemented with a series of loops. First a loop is used to find
all the batches, a loop to pin all the HW contexts, a loop to generate
all the requests, a loop to submit all the requests, a loop to commit
all the requests, and finally a loop to tie the requests to the VMAs
they touch.

A composite fence is also created for the also the generated requests to
return to the user and to stick in dma resv slots.

IGT: https://patchwork.freedesktop.org/patch/447008/?series=93071&rev=1
media UMD: link to come

Signed-off-by: Matthew Brost <matthew.brost@intel.com>