drm/amd/display: Fix incorrect size calculation for loop

...

Memory corruption

In Poppler 0.59.0, memory corruption occurs in a call to Object::streamGetChar in Object.h after a repeating series of Gfx::display, Gfx::go, Gfx::execOp, Gfx::opShowText, and Gfx::doShowText calls aka a Gfx.cc infinite loop...