lcd: restore ability to display 8 bpp BMPs on 16 bpp LCDs

Commit fb6a9aab (LCD: display 32bpp decompressed bitmap image)
broke the check that allowed U-Boot to display 8 bpp BMPs on a 16
bpp LCD screen, effectively turning this feature off.

Restore this feature by changing the check back to the same meaning
it originally had.
To avoid future confusion, the check has also been rephrased to make
its meaning clear.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>