x86/efi/bgrt: Don't ignore the BGRT if the 'valid' bit is 0

Unintuitively, the BGRT graphic is apparently meant to be usable
if the valid bit in not set. The valid bit only conveys
uncertainty about the validity in relation to the screen state.

Windows 10 actually uses the BGRT image for its boot screen even
if not 'valid', for example when the user triggered the boot
menu. Because it is unclear if all firmwares will provide a
usable graphic in this case, we now look at the BMP magic number
as an additional check.

Reviewed-by: Josh Triplett <[email protected]>
Signed-off-by: Môshe van der Sterre <[email protected]>
Signed-off-by: Matt Fleming <[email protected]>
Cc: =?UTF-8?q?M=C3=B4she=20van=20der=20Sterre?= <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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