panfrost: Take into account a index_bias for glDrawElementsBaseVertex calls

panfrost: Take into account a index_bias for glDrawElementsBaseVertex calls

Midgard does not accept a index_bias directly and relies instead on a
bias correction offset (offset_bias_correction) in order to calculate
the unbiased vertex index.

We need to make sure we adjust offset_start and vertex_count in order to
take into account the index_bias as required by a
glDrawElementsBaseVertex call and then supply a additional
offset_bias_correction to the hardware.

Signed-off-by: Rohan Garg <rohan.garg@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>