 Some place in mesa code you are still useing 4096 . 
 I am setting environment EGL_LOG_LEVEL=debug EGL_PLATFORM=drm R600_DEBUG=nodma
, and I am running egltri_screen.    There havn't a tringle display in the
screen center. the message info as follow :

libEGL debug: Native platform type: drm (environment overwrite)
libEGL debug: EGL search path is /usr/lib64/egl
libEGL debug: added egl_dri2 to module array
libEGL debug: the best driver is DRI2
EGL_VERSION = 1.4 (DRI2)
libEGL debug: attribute 0x3033 has an invalid value 0x8
libEGL debug: EGL user error 0x3004 (EGL_BAD_ATTRIBUTE) in eglChooseConfig

EGLUT: failed to choose a config