mesa: Make TexSubImage check negative dimensions sooner.

mesa: Make TexSubImage check negative dimensions sooner.

Two dEQP tests expect INVALID_VALUE errors for negative width/height
parameters, but get INVALID_OPERATION because they haven't actually
created a destination image.  This is arguably not a bug in Mesa, as
there's no specified ordering of error conditions.

However, it's also really easy to make the tests pass, and there's
no real harm in doing these checks earlier.

Fixes:
dEQP-GLES3.functional.negative_api.texture.texsubimage3d_neg_width_height
dEQP-GLES31.functional.debug.negative_coverage.get_error.texture.texsubimage3d_neg_width_height

v2: Drop redundant check (caught by Anuj Phogat).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>