drm/nouveau/core: add firmware handling functions

Add two functions nvkm_firmware_get() and nvkm_firmware_put() to load a
firmware file and free its resources, respectively. Since firmware files
are becoming a necessity for new GPUs, and their location has been
standardized to nvidia/chip/, this will prevent duplicate and
error-prone name-generation code.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>