meta: Remove error checks for texture <-> pixel-buffer transfers that don't belong...

meta: Remove error checks for texture <-> pixel-buffer transfers that don't belong in driver code

The implementation of texture <-> pixel-buffer transfers in drivers common layer
includes certain error checks and argument validation that don't belong there,
considering how the Mesa codebase is laid out. These are higher level
validations that, if necessary, should be performed earlier (i.e, in GL API
entry points).

This patch simply removes these error checks from driver code.

For more information, see discussion at
http://lists.freedesktop.org/archives/mesa-dev/2015-February/077417.html.

Reviewed-by: Laura Ekstrand <laura@jlekstrand.net>