mesa: Separate PBO validation checks from buffer mapping, to allow reuse

mesa: Separate PBO validation checks from buffer mapping, to allow reuse

Internal PBO functions such as _mesa_map_validate_pbo_source() and
_mesa_validate_pbo_compressed_teximage() perform validation and buffer mapping
within the same call.

This patch takes out the validation into separate functions to allow reuse
of functionality by other code (i.e, gl(Compressed)Tex(Sub)Image).

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