mesa: stop using _mesa_error_check_format_type() in glDrawPixels

mesa: stop using _mesa_error_check_format_type() in glDrawPixels

The _mesa_error_check_format_type() function does two things: check
that format/type is legal and check that the destination (or source
buffer for glReadPixels) actually exists.  Just move the relevant
parts of that into _mesa_DrawPixels().

We'll do a similar change in glReadPixels then get rid of the function
altogether.

Signed-off-by: Brian Paul <brianp@vmware.com>