drm: Use DRM_FORMAT_MOD_LINEAR when no other format modifiers are present

The absence of modifiers means "assume linear" not "don't use any modifier". This prevents gallium from using plain resource_create but rather resource_create_with_modifiers.

This unbreaks broken rendering on etnaviv as resource_create assumes it can supertile (which makes sense since e.g. dcss and ipuv3 can handle these formats). However lcdif/mxsfb can't handle this so be explicit about the modifier.

Weston does the same, see drm_plane_populate_formats().

Closes: mesa#4