drm: omapdrm: fb: Simplify mode command checks when creating framebuffer

The hardware requires all planes to have an identical pitch in number of
pixels. Given that all supported formats use the same number of bytes
per pixel in all planes, framebuffer creation checks can be simplified.
The implementations assumes that no format use more than two planes
which is true with the existing hardware.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>