r300: fix texture size handling with size > 2048

r300: fix texture size handling with size > 2048

The in kernel texture check fails because of both
bit11 flags being set on 16x16 textures. It tuns out
that these bits are still set and not cleared in the
pp_txpitch field of the texture. The attached patch
at least helps for this case on my machine. It clears
the bit 11 from the pitch field if the texture is smaller
and masks out that hight bits on the conventional width
and height field.

Fixes bug 24584