r600g: do not use "fast-clear" for small textures (v3)

r600g: do not use "fast-clear" for small textures (v3)

Ported from radeonsi. Improves windowed glxgears ran as

	vblank_mode=0 glxgears -info -geometry 0+0+512+512

from ≈2270 FPS to ≈2360 FPS. Tested with AMD TURKS.

v2: turned out glxgears ignores the option above, the correct way would
be "512x512+0+0". Now it can be seen 512x512 actually loses 30 FPS.
300×300 however wins around a hundred FPS, and to leave some room in
case results may differ for other cards I want not to nitpick in search
of an optimum but to simply leave 300×300 in the code.
v3: remove redundant braces, and try harder for the mail to stick to
the rest of the series.

Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Reviewed-by: Gert Wollny <gw.fossdev@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>