efi-bgrt: Add error handling; inform the user when ignoring the BGRT

Gracefully handle failures to allocate memory for the image, which might
be arbitrarily large.

efi_bgrt_init can fail in various ways as well, usually because the
BIOS-provided BGRT structure does not match expectations.  Add
appropriate error messages rather than failing silently.

Reported-by: Srihari Vijayaraghavan <linux.bug.reporting@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=81321


Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>