mesa: Fix return type of _mesa_get_format_bytes() (#37351)

mesa: Fix return type of  _mesa_get_format_bytes() (#37351)

Despite that negative values aren't sensible here, making this unsigned
is dangerous.  Consider get_pointer_generic, which computes a value of
the form:

    void *base + (int x * int stride + int y) * unsigned bpp

The usual arithmetic conversions will coerce the (x*stride + y)
subexpression to unsigned.  Since stride can be negative, this is
disastrous.

Fixes at least the following piglit tests on Ironlake:

    fbo/fbo-blit-d24s8
    spec/ARB_depth_texture/fbo-clear-formats
    spec/EXT_packed_depth_stencil/fbo-clear-formats

NOTE: This is a candidate for the 7.10 branch.

Reviewed-by: Chad Versace <chad.versace@intel.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>