drm: omapdrm: Use sizeof(*var) instead of sizeof(type) for structures

By linking the sizeof to a variable type the code will be less prone to
bugs due to future type changes of variables.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>