i965: Fix out-of-order sampler unit usage in ARB fragment programs.

i965: Fix out-of-order sampler unit usage in ARB fragment programs.

ARB fragment programs use texture unit numbers directly, unlike GLSL
which has an extra indirection.  If a fragment program only uses one
texture assigned to GL_TEXTURE1, SamplersUsed will only contain a single
bit, which would make us only upload a single surface/sampler state
entry.  However, it needs to be the second entry.

Using _mesa_fls() instead of _mesa_bitcount() solves this.  For ARB
programs, this makes num_samplers the ID of the highest texture unit
used.  Since GLSL uses consecutive integers assigned by the linker,
_mesa_fls() should give the same result as _mesa_bitcount()..

Fixes a regression since 85e8e9e000732908b259a7e2cbc1724a1be2d447,
which caused GPU hangs in ETQW (and probably others), as well as
breaking piglit test fp-fragment-position.

v2: Add a comment, as suggested by Matt.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54098
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54179
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: meng <mengmeng.meng@intel.com>