drm/format-helper: Only advertise supported formats for conversion

drm_fb_build_fourcc_list() currently returns all emulated formats
unconditionally as long as the native format is among them, even though
not all combinations have conversion helpers. Although the list is
arguably provided to userspace in precedence order, userspace can pick
something out-of-order (and thus break when it shouldn't), or simply
only support a format that is unsupported (and thus think it can work,
which results in the appearance of a hang as FB blits fail later on,
instead of the initialization error you'd expect in this case).

Add checks to filter the list of emulated formats to only those
supported for conversion to the native format. This presumes that there
is a single native format (only the first is checked, if there are
multiple). Refactoring this API to drop the native list or support it
properly (by returning the appropriate emulated->native mapping table)
is left for a future patch.

The simpledrm driver is left as-is with a full table of emulated
formats. This keeps all currently working conversions available and
drops all the broken ones (i.e. this a strict bugfix patch, adding no
new supported formats nor removing any actually working ones). In order
to avoid proliferation of emulated formats, future drivers should
advertise only XRGB8888 as the sole emulated format (since some
userspace assumes its presence).

This fixes a real user regression where the ?RGB2101010 support commit
started advertising it unconditionally where not supported, and KWin
decided to start to use it over the native format and broke, but also
the fixes the spurious RGB565/RGB888 formats which have been wrongly
unconditionally advertised since the dawn of simpledrm.

Fixes: 6ea966fc ("drm/simpledrm: Add [AX]RGB2101010 formats")
Fixes: 11e8f5fd ("drm: Add simpledrm driver")
Cc: stable@vger.kernel.org
Signed-off-by: Hector Martin <marcan@marcan.st>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221027135711.24425-1-marcan@marcan.st