drm/i915: Use devm_drm_dev_alloc

Luckily we're already well set up in the main driver, with
drm_dev_put() being the last thing in both the unload error case and
the pci remove function.

Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-39-daniel.vetter@ffwll.ch