gcx: validate dest rect after clipping

According to bltsville spec, rectangles should not be checked
for validity until clipping has been applied.  So we must clip
the dest rect before validating it.  Removed the error checking
from the generic rect converter macro.  Added error checking
specifically for clip rect and dest rect, post clipping. Proper
validity checking of other rects is TBD.

Change-Id: I13854c150b2549fd5a91895c7d594d00045affec
Signed-off-by: Craig Stout <craig.stout@ti.com>