drm/radeon: Set depth on low mem to 16 bpp instead of 8 bpp

drm/radeon: Set depth on low mem to 16 bpp instead of 8 bpp

The radeon driver reduces the framebuffer resolution to 8bpp if a
device with less than 32MB VRAM is found.  This causes the framebuffer
to run in 8 bit paletted mode.  For a text console this is not an
issue as 256 different colors is more than one gets on a VGA text
console.  However this leads to a poor 8bit pseudo-color visual when
running X on fbdev, too, which is quite ugly.

In this patch, we try to give some moderate compromise: limit the
framebuffer bpp to 8 only when VRAM is 8MB or less, and use 16 bpp
otherwise for 32MB or less VRAM.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>