drm/etnaviv: move GPU linear window to end of DMA window

If the end of the system DMA window is farther away from the start of
physical RAM than the size of the GPU linear window, move the linear
window so that it ends at the same address than the system DMA window.

This allows to map command buffer from CMA, which is likely to reside
at the end of the system DMA window, while also overlapping as much
RAM as possible, in order to optimize regular buffer mappings through
the linear window.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>