Fix regression in reading certain compressed bmp images

Fix regression in reading certain compressed bmp images

For BITFIELDS compressed images, the image data may be preceded by
bitmask fields. The file positioning code that should make sure those
were read correctly was erroneously included in the block of code that
was moved in 6f1b82fccdaf202856dcc6510c16b0531680fe23.

Task-number: QTBUG-45559
Task-number: QTBUG-40890
Done-with: Eirik Aavitsland
Change-Id: Id2b3ce078f67ac6ebf75ab0cc463dc719af83393
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>