drm/i915/gt: Allocate i915_fence_reg array

Since the number of fence regs can vary dramactically between platforms,
allocate the array on demand so we don't waste as much space.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316113846.4974-4-chris@chris-wilson.co.uk