Sometimes it would be most enlightening to debug systems by replacing
the VBT to be used. For example, in the referenced bug the BIOS provides
different VBT depending on the boot mode (UEFI vs. legacy). It would be
interesting to try the failing boot mode with the VBT from the working
boot, and see if that makes a difference.

Add a module parameter to load the VBT using the firmware loader, not
unlike the EDID firmware mechanism.

As a starting point for experimenting, one can pick up the BIOS provided
VBT from /sys/kernel/debug/dri/0/i915_opregion/i915_vbt.

v2: clarify firmware load return value check (Bob)

v3: kfree the loaded firmware blob

References: https://bugs.freedesktop.org/show_bug.cgi?id=97822#c83Reviewed-by: NBob Paauwe <bob.j.paauwe@intel.com>
Acked-by: NDaniel Vetter <daniel@ffwll.ch>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170817115209.25912-1-jani.nikula@intel.com