pvr: Change push_constants_shader_stages to type pvr_stage_allocation

pvr: Change push_constants_shader_stages to type pvr_stage_allocation

Previously the code was saving the mask as a VkShaderStageFlags
but when allocating shareds it checked against pvr_stage_allocation.
This causes problems as only the vertex bit matches the
VkShaderStageFlagBits so the push constants utilized in fragment
shaders weren't picked up properly.

Signed-off-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22731>