da8xx-fb: allow frame to complete after disabling LCDC

Wait for active frame transfer to complete after disabling LCDC.
At the same this wait is not be required when there are sync and
underflow errors.
Patch applies for revision 2 of LCDC present am335x.
More information on disable and reset sequence can be found in
section 13.4.6 of AM335x TRM @www.ti.com/am335x.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>